Sounds like a job for DRI3K.
On the client side, I believe the compositing window manager is responsible for this, and in the case of wiki/ClientRendering it should be pretty easy:
See also: Adding synchronization to the WM spec (wm-spec-list)
We should already have mostly tear free frames thanks to the "flush" screen update flag.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/386