Thanks for your support 
I have taken some time to be able to passthrough my own GPU to sys-gui-gpu to be able to make use of this nice work myself 
This being slowed by build times, in turn has brought me to see how we could improve the Qubes build system ā but it appears not a good moment to work on big changes there, so Iām refocusing on investigating just activating various acceleration options in Mock (ccache, buildchroot caching and such) to make stubdom rebuild times more acceptable.
With a bit more work and some luck I may end up having this sys-gui-gpu working (and this, as being just a personal side-project, can take some time).
But once these digressions get done, I intend to look first into how to leverage the Qubes GUI protocol to avoid the double-window issue (which should make the whole thing realistic to use, even though a couple of important things will still need to be fixed before calling it a first release).