I don't know anything about bevy but yeah, that looks like it would be a challenge for the models. In this particular case I'd tell the model how I wanted it to work - rather than "Add a button to the left panel that prints "Hello world" when pressed" I'd say something more like (I'm making up these details): "Use the bevy:Panel class with an inline callback to add a button to the bottom of the left panel".

Or I'd more likely start by asking for options: "What are some options for adding a button to that left panel?" - then pick one that I liked, or prompt it to use an approach it didn't suggest.

After it delivered code, if I didn't like the code it had used I'd tell it: "Don't use that class, use X instead" or "define a separate function for that callback" or whatever.