the prefdm trick will only work if you use xdm/gdm/kdm. If you use startx, a change in prefdm won't affect X at all. You can do something like "LD_ASSUME_KERNEL=2.2.5 startx" (or edit /usr/X11R6/bin/startx, it's a shell script) to get the same effect without xdm.
The 2.2.5 might make more sense if you look at it as "kernel version that didn't have the features required for supporting the features that don't work". Anyway, it does the trick.
Might be, that even something "newer" would work too, 2.2.5 is the standard to make random binary-only software work (eg. older versions of java), which is why I chose it. It'd also disable less features in glibc, which would probably be advantageous.
Easiest way is probably starting up X with 2.2.5, then trying glxinfo with various values until it segfaults. Can't easily test it currently myself, alas.