That seems like it could be interesting. It sounds like you would need to have part of a library store and diff the tree state, and then send updates based on that? You could use that part for immediate mode GUIs, and then retained mode GUIs could skip it and maintain their own trees, and just deal with the protocol, maybe even the raw protobuf bindings or whatever it is you choose.

The Linux desktop environments could really use more people working on the accessibility stack, it's really outdated at the moment.