Comment # 4 on bug 65192 from Michel Dänzer

(In reply to comment #3)
> R600_HYPERZ=0 /usr/bin/xscreensaver-demo
> 
> and switching between screensavers crashed as often as without R600_HYPERZ=0.

Beware that AFAIK the screensaver hacks aren't spawned from the
xscreensaver-demo process but from the xscreensaver daemon process. Did you
confirm that the latter saw the environment variable?