In Unity 2019.1 Unity added a new custom EditorTool API, heres some example code showing some tools we could make with it!

PhysicsTool.cs

// Put me in an editor folder!
using System.Collections;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using UnityEditor;
using UnityEditor.EditorTools;
using UnityEngine;

[EditorTool("PhysicsDrop Tool", typeof(Rigidbody))]
public class PhysicsTool : EditorTool
{
    GUIContent cachedIcon;

    // NOTE: as were caching this, unity will serialize it between compiles! so if we want to test out new looks,
    // just return the new GUIContent and bypass the cache until were happy with the icon...
    public override GUIContent toolbarIcon
    {
        get
        {
            if (cachedIcon == null)
                cachedIcon = EditorGUIUtility.IconContent("PhysicMaterial Icon", "|PhysicsDrop Tool");
            return cachedIcon; 
        }
    }

    private List<RigidbodyCache> bodyCache = new List<RigidbodyCache>();
    public override void OnToolGUI(EditorWindow window)
    {
        
        // make sure we have objects to work with...
        if (Selection.activeTransform == null)
            return;
        
        // lets convert the selected objects postion into a screen position...
        Vector3 pos = Selection.activeTransform.position;
        Vector3 screenPoint = SceneView.lastActiveSceneView.camera.WorldToScreenPoint(pos);
        
        // ... if its visible, lets draw some GUI over it
        if (screenPoint.z > 0)
        {
            // Make sure we tell Unity were drawing some UI
            Handles.BeginGUI();

            // lets call into PixelsToPoints, this takes editor GUI scale/high DPI screens into account to make sure the 
            // UI aligns correctly
            screenPoint = EditorGUIUtility.PixelsToPoints(SceneView.lastActiveSceneView.camera.WorldToScreenPoint(pos));
            
            Vector2 buttonSize = new Vector2(100,20);
            float screenHeight = SceneView.currentDrawingSceneView.position.size.y;
            Vector2 buttonPos = new Vector2(screenPoint.x, screenHeight - (screenPoint.y));
            
            // Actually draw the button
            if (GUI.Button(new Rect(buttonPos, buttonSize), "Simulate"))
            {
                // first, lets grab all the rigidbodies were working on...
                var selectedBodies = new List<Rigidbody>();
                foreach (var gameObject in Selection.gameObjects)
                {
                    var body = gameObject.GetComponent<Rigidbody>();
                    if (body != null)
                    {
                        selectedBodies.Add(body);
                    }
                }
                
                // then lets grab all the rigidbodies in the scene and cache their positions
                var bodies = FindObjectsOfType<Rigidbody>();
                bodyCache.Clear();
                foreach (var body in bodies)
                {
                    bodyCache.Add(new RigidbodyCache(body));
                }
                
                // Tell the undo system were about to modify our selected objects.
                Undo.RecordObjects(Selection.transforms, "Physics Simulation...");

                // Annnd finally lets tick the physics engine!!
                Physics.autoSimulation = false;
                for (int i = 0; i < 600; i++)
                {
                    Physics.Simulate(Time.fixedDeltaTime);
                }
                Physics.autoSimulation = true;  
                
                // Since Physics.Simulate effected everything in the scene, lets loop over the cached copies
                // of all the rigid bodies and reset the positions and rotations of everything that wasnt selected!
                // We should also make sure to clear accumulated velocities, so they dont carry over
                foreach (var body in bodyCache)
                {
                    if (!selectedBodies.Contains(body.Rigidbody))
                    {
                        var transform = body.Rigidbody.transform;
                        transform.position = body.Position;
                        transform.rotation = body.Rotation;
                    }

                    body.Rigidbody.velocity = Vector3.zero;
                    body.Rigidbody.angularVelocity = Vector3.zero;
                }
            }
            
            // Finally lets tell Untiy we finished our UI
            Handles.EndGUI();
        }
    }
}

public struct RigidbodyCache
{
    public Rigidbody Rigidbody;
    public Vector3 Position;
    public Quaternion Rotation;

    public RigidbodyCache(Rigidbody rigidbody)
    {
        this.Rigidbody = rigidbody;
        this.Position = rigidbody.position;
        this.Rotation = rigidbody.rotation;
    }
}

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Always thankful for your support, Lotte💖