Disabling an Xorg server's extension at runtime (for new clients)?

I have an old Xorg server whose ["Render" extension seems to be buggy (if enabled, most of the fonts are invisible, unless hovering over them -- after upgrading the packages with X clients and libraries)](https://bugzilla.altlinux.org/show_bug.cgi?id=29956) . So, the problem is solved for new instances of the Xorg server by adding to /etc/X11/xorg.conf: Section "Extensions" Option "RENDER" "Disable" EndSection But what if I want to continue an X session, but make new clients not use the extension? (Of course, logically, [old clients can't be made not to use it](https://unix.stackexchange.com/a/4288/4319) ; but what about new ones?) Some options are: 1. Can I turn one of the server's extension off on the fly, while the server is running? 2. Can I tell the clients not to use it (e.g., those written with a specific toolkit)? 3. IS there an "X proxy" which could be used for my goal: the proxy could simply not advertise that it has a "Render" extension, or translate the requests that involve it, and forward everything to the "physical" X server, and my new clients would then connect to the proxy's DISPLAY.