I can help. I’ll send a PM.
EDIT: Turns out what he was asking and what I thought he was asking were entirely different. He wants a way to make it possible to use a handlebar placeholder anywhere in existing themes. I am not aware of a way to do that, and, as far as I can tell, neither is anyone else.
What I thought he wanted was to add the user’s email address to the current_user serializer. That can be done for the proposed budget.