HI,
using debian-8.1.0-powerpc-xfce-CD-1.iso
I managed to get debian 8.1 installed on macG5 today after 3 attempts -
1) I ran into a problem the first time round when trying to install more software - I chose all 3 options provided by the installer ie.Debian Destop environment, xfce and system utilities - half way through it ran into a problem reading from the disk so I had to reboot
2) I entered rescue at the boot prompt and started form there but there was obviously a fault with the CD so I burned a new copy, got the base system installed, installed yaboot and rebooted but my username/passwords didn't work so again I rebooted
3) I entered install again at the boot prompt and installed the base system -skipped installing any extra software, installed yaboot and now I have a working base system