Creating a complex prefab during asset postprocessing

In OnPostprocessModel, I want to prepare a GameObject that is parented to another GameObject, and then create a prefab out of that. This works fine, but it leaves an instance in the scene. When I try to destroy the object after creating the prefab, the editor crashes. Here's the code:

public class MyAssetPostprocessor : AssetPostprocessor
{
    public void OnPostprocessModel(GameObject gameObject)
    {
        // create an empty gameobject and add the mesh as a child
        GameObject prefabRoot = new GameObject("foo");
        gameObject.transform.parent = prefabRoot.transform;

        // create the prefab, and connect the dummy prefab root to it
        Object prefab = EditorUtility.CreateEmptyPrefab("Assets/test.prefab");
        EditorUtility.ReplacePrefab(prefabRoot, prefab, ReplacePrefabOptions.ConnectToPrefab);

        // cleanup the gameobject instance in the scene
        Object.DestroyImmediate(prefabRoot);
    }
}

I'm not convinced I'm heading down the right path. What's the correct way to create complex prefabs during asset postprocessing, involving the asset which is being imported?