From the document ‘Create a Custom Inspector’, there has a great way to reference a VisualTreeAsset by reference without hard code the asset path.
It work in Editor and EditorWindow. But it not work in a PropertyDrawer. It could be good if support it.
PropertyDrawer is not a MonoScript type, this is only supported for MonoScript types. This is Unity itself, not UI Toolkit. There’s nothing we can do about it, sadly!
I would also like to see this! Was trying to follow the getting started with UI Toolkit guide in the Unity documentation and got stuck in this part.
@JuliaP_Unity whom could we ask about making this change to PropertyDrawer? It’s not like you guys aren’t in the same company 
that person has to exist!
@marcospgp you can submit feedback through the Help > Report a Bug tool, or by submitting feedback on the Unity Roadmap
There you can read about all upcoming changes as well.
For this type of request, the “Editor” roadmap is probably the most appropriate.
This is the type of thing that has a lot more value coming from users than coming from us, internally 