UdonSharpの外部アセットへの参照を突っ込むフィールドにつけるとゆるく型チェックしてくれるやつ

LooselyTypedAttribute.cs

using UnityEngine;

namespace Silksprite.Attributes
{
    public class LooselyTypedAttribute : PropertyAttribute
    {
        public readonly string TypeName;

        public LooselyTypedAttribute(string typeName)
        {
            TypeName = typeName;
        }
    }
}

LooselyTypedPropertyDrawer.cs

using Silksprite.Attributes;
using UnityEditor;
using UnityEngine;

namespace Silksprite.PropertyDrawers
{
    [CustomPropertyDrawer(typeof(LooselyTypedAttribute))]

    public class LooselyTypedPropertyDrawer : PropertyDrawer
    {
        string TypeName => ((LooselyTypedAttribute)attribute).TypeName;

        object FindWrongProperty(SerializedProperty property)
        {
            bool CheckWrongProperty(SerializedProperty item)
            {
                var value = item.objectReferenceValue;
                if (value == null) return false;
                return value.GetType().Name != TypeName;
            }

            if (property.isArray)
            {
                for (var i = 0; i < property.arraySize; i++)
                {
                    var elementProperty = property.GetArrayElementAtIndex(i);
                    if (CheckWrongProperty(elementProperty)) return elementProperty.objectReferenceValue;
                }
            }
            else
            {
                if (CheckWrongProperty(property)) return property.objectReferenceValue;
            }

            return null;
        }

        GUIContent ErrorMessage(SerializedProperty property)
        {
            var wrongProperty = FindWrongProperty(property);
            return wrongProperty == null ? null : new GUIContent($"Object is not {TypeName}, found {wrongProperty.GetType().Name}");
        }

        public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
        {
            var errorMessage = ErrorMessage(property);
            if (errorMessage == null)
            {
                EditorGUI.PropertyField(position, property, label, true);
                return;
            }

            var rect = position;
            rect.height = base.GetPropertyHeight(property, label);
            EditorGUI.PropertyField(rect, property, label, true);

            var messageHeight = EditorStyles.helpBox.CalcHeight(errorMessage, EditorGUIUtility.currentViewWidth);
            position.yMin = position.yMax - messageHeight; 
            EditorGUI.HelpBox(position, errorMessage.text, MessageType.Error);
        }

        public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
        {
            var errorMessage = ErrorMessage(property);
            if (errorMessage == null)
            {
                return base.GetPropertyHeight(property, label);
            }
            var messageHeight = EditorStyles.helpBox.CalcHeight(ErrorMessage(property), EditorGUIUtility.currentViewWidth);
            return base.GetPropertyHeight(property, label) + EditorGUIUtility.standardVerticalSpacing + messageHeight;
        }
    }
}

Usage.cs

[Header("QvPen_PenManager Component:")]
[SerializeField] [LooselyTyped("QvPen_PenManager")] UdonSharpBehaviour[] penManagers;
[Header("QvPen_EraserManager Component:")]
[SerializeField] [LooselyTyped("QvPen_EraserManager")] UdonSharpBehaviour[] eraserManagers;