If we can't change the graphics subsystem's dependency on being able to send requests to the X11 server, then perhaps we could intercept them and try to restrict them to operating only on resources (drawables?) that it owns. At the moment, I have no idea how much effort this might require. It would be helpful to have some data — for example, by intercepting the traffic and logging information about the requests (opcodes used, values of certain fields, etc.). The machine-readable protocol descriptions in the xcb-proto project (git://anongit.freedesktop.org/git/xcb/proto) may be helpful here.

http://xtrace.alioth.debian.org/ should be able to give you the logging that you need.

Moving to sblc4 which deals with this issue.

This is even without opening a proper content tab.

Mentioning this because it'll probably be annoying to debug if it matters: X11 can do fd-passing, so the proxy may need to receive and forward SCM_RIGHTS. See also: https://keithp.com/blogs/fd-passing/