On pre-r300 Radeon GPUs, kernel modesetting (KMS) breaks hardware acceleration on Apple machines. After starting xorg, the screen flickers endlessly until the computer is shut down unless Option "NoAccel" "true" is specified in the xorg.conf file. This makes any pre-G4 PowerPC machine unusable with any newer version of a Linux distribution.
Unfortunately, UMS support has been deprecated in both Xorg and the Radeon driver. But downgrading the xserver-xorg package to the 1.16.4 version, and the xorg-server-video-ati package to 6.14.6 would make the bug dissapear, and would give excellent 2D acceleration, if coupled with Linux 4.4, the last kernel with UMS support.
I have tested this solution in Debian, downgrading the xorg packages to their Wheezy versions with some apt trickery, and it works. More information can be found here: https://forums.macrumors.com/threads/making-g3s-and-radeon-graphics-great-again.2191877/
I propose the creation of an xorg-server-old package and a xorg-server-video-ati-old package for the users of these old Radeon cards. As the Xorg codebase is pretty stable, backporting security fixes from newer versions should be relatively easy.
If noone has the time to do this, I could try doing it myself, but I would need help with building apk packages.