How can I restrict the object selection to prefabs only?

Is there any way to constraint the possible selection of GameObjects in an ObjectField (EditorGUILayout.ObjectField) to Prefabs only?

Just set allow scene objects to false? That will limit it to assets (i.e. not object in the scene).

You can determine if an object is a prefab (in the editor not at runtime… but you are working in the editor) using this:
How to know if a GameObject is a prefab?. Since ObjectField doesn’t have a way to edit the list or do better filtering you’d have to roll your own object selector (other than object type and prefab isn’t a type).