Hey, don't know if you managed to figure this out, but I think CreateEditor or CreateCachedEditor are what you're after. CreateCachedEditor seemed to be the cleanest and simplest implementation.

So if you have a MonoBehaviour:

 using UnityEngine;
 
 public class MyBehaviour : MonoBehaviour
 {
     public MyAsset MemberAsset;
     
     // Note: assets don't need to be public for it to work
     [SerializeField] private MyAsset _memberAsset;
 }

Where MyAsset derives from ScriptableObject (for example -- but any Object should be okay).

You could create something like this (for example):

 using UnityEngine;
 using UnityEditor;
 
 [CustomEditor(typeof(MyBehaviour))]
 public class MyBehaviourEditor : Editor
 {
     private Editor _editor;
     
     public override void OnInspectorGUI()
     {
         serializedObject.Update();
         
         var myAsset = serializedObject.FindProperty("MemberAsset");
         CreateCachedEditor(myAsset.objectReferenceValue, null, ref _editor);
         _editor.OnInspectorGUI();
         
         serializedObject.ApplyModifiedProperties();
     }
 }

(Obviously, you could play with the other functions that can be called on editors here too.)

Note that, it will draw the inspector just as it would if you had it in the inspector. You may not want this (e.g. may not want the script field to display), so you can supply a custom editor that will do the job. In fact, you could write as many editors as you wanted and select which editor will be used as the second parameter (we passed null here, which will just give you the default editor for that Object).

Hope that's what you were after =).

If anyone's still wondering and wants a general solution that works for every scriptable object, without needing to write custom inspectors, you can take a look at the post over here: https://forum.unity3d.com/threads/editor-tool-better-scriptableobject-inspector-editing.484393/

It takes a custom property drawer for ScriptableObjects to display every derived class within the main inspector of the MonoBehaviour.

https://gist.github.com/tomkail/ba4136e6aa990f4dc94e0d39ec6a058c

in case someone still looking for a way to do this

One way for such a simple class would be to add getters and setters to MyBehavior that would access the memberAssets properties.

     public int param1
     {
         get
         {
             return memberAsset.param1;
         }
         
         set
         {
             memberAsset.param1 = value;
         }
     }

Mark MyAsset as serializable and it will show up in the inspector nicely.

 [System.Serializable]
 public class MyAsset : ScriptableObject
 {
 }