An example of a custom ObjectPreview rendering out a SkinnedMesh for Unity Editor C#... I used it for facial expressions and blendshape editing, you might want to use it for something else. I hereby place it under MIT License. Full write-up here: http://www.blog.radiator.debacle.us/2016/06/working-with-custom-objectpreviews-and.html

SkinnedMeshObjectPreviewExample.cs

using UnityEngine;
using UnityEditor;
using System;
using System.Collections;

[CustomPreview(typeof(YourCustomScriptableObject))] // THIS IS VERY IMPORTANT, this is so the editor knows what this is supposed to be previewing at all
public class SkinnedMeshObjectPreviewExample : ObjectPreview {

	PreviewRenderUtility m_PreviewUtility;

	GameObject previewPrefab, previewInstance;
	SkinnedMeshRenderer skinMeshRender, eyeballRender;
	Mesh previewMesh, previewMeshEyeball;
	Material previewMaterial;

	static Vector2 previewDir = new Vector2(-180f, 0f);
	static float lerpSpeed = 0.5f;

	// very important to override this, it tells Unity to render an ObjectPreview at the bottom of the inspector
	public override bool HasPreviewGUI() { return true; }

	private void Init() {
		if (this.m_PreviewUtility == null) {
			this.m_PreviewUtility = new PreviewRenderUtility();
			this.m_PreviewUtility.m_CameraFieldOfView = 27f;
			RefreshPreviewInstance();
		}
	}

	// the main ObjectPreview function... it's called constantly, like other IMGUI On*GUI() functions
	public override void OnPreviewGUI(Rect r, GUIStyle background) {
		// if this is happening, you have bigger problems
		if (!ShaderUtil.hardwareSupportsRectRenderTexture) {
			if (Event.current.type == EventType.Repaint) {
				EditorGUI.DropShadowLabel(new Rect(r.x, r.y, r.width, 40f), "Mesh preview requires\nrender texture support");
			}
			return;
		}
		Init();
		previewDir = Drag2D(previewDir, r);
		if (Event.current.type != EventType.Repaint) { // if we don't need to update yet, then don't
			return; 
		}
		m_PreviewUtility.BeginPreview(r, background); // set up the PreviewRenderUtility's mini internal scene
		DoRenderPreview();
		Texture image = m_PreviewUtility.EndPreview(); // grab the RenderTexture resulting from DoRenderPreview() > RenderMeshPreview() > PreviewRenderUtility.m_Camera.Render()
		GUI.DrawTexture(r, image, ScaleMode.StretchToFill, false); // draw the RenderTexture in the ObjectPreview pane
		EditorGUI.DropShadowLabel(new Rect(r.x, r.y, r.width, 40f), target.name);
	}

	private void DoRenderPreview() {
		if ( skinMeshRender == null ) {
			RefreshPreviewInstance();
		}

		// very important: we have to call BakeMesh, to bake that animated SkinnedMesh into a plain static Mesh suitable for Graphics.DrawMesh()
		previewMesh = new Mesh();
		previewMeshEyeball = new Mesh();
		skinMeshRender.BakeMesh( previewMesh );
		eyeballRender.BakeMesh( previewMeshEyeball );

		// now, actually render out the RenderTexture
		RenderMeshPreview( previewMesh, m_PreviewUtility, previewMaterial, previewDir, -1);
	}
	 
	void RefreshPreviewInstance () {
		// if we already instantiated a PreviewInstance previously but just lost the reference, then use that same instance instead of making a new one
		var oldInstance = GameObject.Find( "FaceMorphemePreviewInstance" );
		if ( oldInstance != null ) {
			previewInstance = oldInstance;
		} else { // no previous instance detected, so now let's make a fresh one
			// very important: this loads the PreviewInstance prefab and temporarily instantiates it into PreviewInstance
			previewPrefab = (GameObject)AssetDatabase.LoadAssetAtPath<GameObject>("Assets/gayclub/prefabs_npc/facepreview.prefab");
			previewInstance = (GameObject)GameObject.Instantiate( previewPrefab, previewPrefab.transform.position, previewPrefab.transform.rotation );
			previewInstance.name = "FaceMorphemePreviewInstance";
			// HideFlags are special editor-only settings that let you have *secret* GameObjects in a scene, or to tell Unity not to save that temporary GameObject as part of the scene
			previewInstance.hideFlags = HideFlags.DontSaveInEditor | HideFlags.DontSaveInBuild; // you could also hide it from the hierarchy or inspector, but personally I like knowing everything that's there
		}
		var sRenders = previewInstance.GetComponentsInChildren<SkinnedMeshRenderer>();
		// when instantiating stuff in the scene with HideFlags, that stuff often gets broken when loading a new scene...
		if ( sRenders == null || sRenders.Length == 0 ) { // so I setup a check to detect that case and clean-up
			Debug.Log("cleaning up leftover PreviewInstance!");
			OnDestroy();
			RefreshPreviewInstance();
			return;
		} else { // if nothing is wrong, then we proceed normally
			skinMeshRender = sRenders[0];
			eyeballRender = sRenders[1];
		}
		previewMaterial = skinMeshRender.sharedMaterial; // use sharedMaterial or else you will instantiate a new material
	}

	void RenderMeshPreview (Mesh mesh, PreviewRenderUtility previewUtility, Material material, Vector2 direction, int meshSubset ) {
		if (mesh == null || previewUtility == null) {
			return;
		}
		// Measure the mesh's bounds so you know where to put the camera and stuff
		Bounds bounds = mesh.bounds;
		float magnitude = bounds.extents.magnitude;
		float distance = 4f * magnitude;

		// setup the ObjectPreview's camera
		previewUtility.m_Camera.backgroundColor = Color.gray;
		previewUtility.m_Camera.clearFlags = CameraClearFlags.Color;
		previewUtility.m_Camera.transform.position = new Vector3( 0f, 0.85f, -0.5f); // this used to be "-Vector3.forward * num" but I hardcoded my camera position instead
		previewUtility.m_Camera.transform.rotation = Quaternion.identity;
		previewUtility.m_Camera.nearClipPlane = 0.3f;
		previewUtility.m_Camera.farClipPlane = distance + magnitude * 1.1f;

		// figure out where to put the model
		Quaternion quaternion = Quaternion.Euler(0f, direction.x, 0f); // this used to have "Quaternion.Euler(direction.y, 0f, 0f) * " to apply pitch rotation as well, but I didn't need it
		Vector3 pos = quaternion * -bounds.center;

		// we are technically rendering everything in the scene, so scene fog might affect it...
		bool fog = RenderSettings.fog; // ... let's remember the current fog setting...
		Unsupported.SetRenderSettingsUseFogNoDirty(false); // ... and then temporarily turn it off

		// submesh support, in case the mesh is made of multiple parts
		int subMeshCount = mesh.subMeshCount;
		if (meshSubset < 0 || meshSubset >= subMeshCount) {
			for (int i = 0; i < subMeshCount; i++) {
				// PreviewRenderUtility.DrawMesh() actually draws the mesh
				previewUtility.DrawMesh(mesh, pos, quaternion, material, i);
				previewUtility.DrawMesh(previewMeshEyeball, pos, quaternion, material, i); // my model's eyeballs are a separate mesh, so I have to draw that too
			}
		} else {
			// no submeshes here so let's just draw it normally
			previewUtility.DrawMesh(mesh, pos, quaternion, material, meshSubset);
			previewUtility.DrawMesh(previewMeshEyeball, pos, quaternion, material, meshSubset);
		}
		// VERY IMPORTANT: this manually tells the camera to render and produce the render texture
		previewUtility.m_Camera.Render();

		// reset the scene's fog from before
		Unsupported.SetRenderSettingsUseFogNoDirty(fog);
	} 

	// this is where you draw any settings or controls above the ObjectPreview
	public override void OnPreviewSettings() {
		GUILayout.Label( "LerpSpeed: " );
		lerpSpeed = EditorGUILayout.Slider( lerpSpeed, 0.1f, 0.5f, GUILayout.Width( 100f ) );
	}

	// cleanup stuff so we don't leak everywhere
	public void OnDestroy() {
		Debug.Log("FaceMorphemePreview OnDestroy()");
		if (this.m_PreviewUtility != null) {
			this.m_PreviewUtility.Cleanup();
			this.m_PreviewUtility = null;
		}
		if ( previewInstance != null ) {
			GameObject.DestroyImmediate( previewInstance );
		}
	}

	// from http://timaksu.com/post/126337219047/spruce-up-your-custom-unity-inspectors-with-a
	// this is our inlined version of EditorGUIUtility.Draw2D(), which is internal or something
	public static Vector2 Drag2D(Vector2 scrollPosition, Rect position) {
		int controlID = GUIUtility.GetControlID("Slider".GetHashCode(), FocusType.Passive);
		Event current = Event.current;
		switch (current.GetTypeForControl(controlID)) {
		case EventType.MouseDown:
			if (position.Contains(current.mousePosition) && position.width > 50f) {
				GUIUtility.hotControl = controlID;
				current.Use();
				EditorGUIUtility.SetWantsMouseJumping(1);
			}
			break;
		case EventType.MouseUp:
			if (GUIUtility.hotControl == controlID) {
				GUIUtility.hotControl = 0;
			}
			EditorGUIUtility.SetWantsMouseJumping(0);
			break;
		case EventType.MouseDrag:
			if (GUIUtility.hotControl == controlID) {
				scrollPosition -= current.delta * (float)((!current.shift) ? 1 : 3) / Mathf.Min(position.width, position.height) * 140f;
				scrollPosition.y = Mathf.Clamp(scrollPosition.y, -90f, 90f);
				current.Use();
				GUI.changed = true;
			}
			break;
		}
		return scrollPosition;
	}

}