Ok, got myself a Radeon X1600 Pro (klick) which works perfectly allright in MorphOS. And in contrast to my flashed X1900 XT it allows basic Linux usage (console + Xorg via fbdev). Glamour-accelerated Xorg actually loads without crashing the G5 but the screen output is garbled. Filed a bug about that if someone wants to do further research (klick).

So it looks like I can finally get this G5 11,2 going! Only missing a network card now, to get it online and register.

Edit: Realized the X1600 Pro runs well in Linux with xf86-video-ati driver + EXA acceleration. 2D-accelerated desktop & video playback works. So the perfect card for me! OpenGL has glitches however (klick).