As @robi mentioned, this is one of the features at the top of our to-do-list, so please stay tuned.
Now, most of the functionality required to make this setup work should be already there. Actually, i did a prototype of this a few months ago and don't remember seeing any blockers. I installed Guacamole on the host (through their containerized approach), and then installed xfce4 and tightvncserver within a sysbox container. That's pretty much it. Users would then interact with the Guacamole server (web-based), and reach their sysbox-based environments through the Guacamole interface.
The work that is pending is mainly on the hw-acceleration area, so that we allow GUI apps seating in nested containers (levels > 1), to talk to the hosts' GPUs; apps running at the sysbox container level (level == 1) should be already able to talk to the GPUs.
Please ping me if any of you is interested in trying / documenting this setup, i'd be happy to help.