Back to topic. And a success story (do not count the number of attempts before I got this far).
root@AppelSapje:/proc# cat cpuinfo (stripped)
cpu : 7400, altivec supported / revision: 2.7 (pvr 000c 0207)
clock : 350.000000MHz
platform : PowerMac
model : PowerMac3,1
motherboard : PowerMac3,1 MacRISC2 MacRISC Power Macintosh
detected as : 65 (PowerMac G4 AGP Graphics)
Memory : 1088 MB
pmac-generation : NewWorld
iMac G4 with iOS 10.4 (outdated) installed. So how can I revive this old apple computer without being too outdated? Install mintppc.
So the succesfull attempt was: put mini.iso on USB stick initialized on iOS 10.4. (actually I downloaded the netinstall image [20120225-00:06] and dd'd it from OpenSuse).
Boot from stick, as installation instructions. But first two separate remarks.
The 85% stop problem is most probably caused by the way the "bootup file" initrd.img is put together. This is caused, I tend to assume, by the fact that the file is created in the default installation procedure with "all drivers available". To work around this problem, go for the "manual" installation.
There is one more problem on my machine, that is the keyboard and mouse were irresponsive after "succesful" installation. So I had to be able to reach the machine on command-line interface to repair this issue manually (if no other machine at hand, it can also be repaired afterwards using the "boot> rescue url=mintppc.org" method).
So having said this: first boot in \\yaboot but then at the boot> prompt enter:
boot> expert url=mintppc.org
Follow instructions, work your way down the installation menu. In the step "Download debconf preconfiguration file" the mintppc.org installation automation is loaded. Select the "wheezy - testing" version. In the "Installer components" I added "Network console / Openssh-client-udeb / Rescue-mode". Then I was able to login to the installation using ssh. Completed the basic installation and Package selection (which is automated in the mintppc file).
After reboot the mouse and keyboard were "locked", even the mouse red light was off. Via ssh logged on as root (you have to allow this in the installation), and added the last three lines to the /etc/modprobe.d/blacklist.local.conf file.
root@AppelSapje:/etc/modprobe.d# cat blacklist.local.conf
# Local module settings
# Created by the Debian installer
# added to avoid system check on bootup
Maybe these problems can be fixed in the installation procedure, but I succeeded to install.