The victim was a PowerMac 9500 with a 300MHz G3 CPU, 200Mb RAM, a 9G HDD (with OS9 installed), and a 2G HDD (blank).
I hooked up a PowerBook to see the serial console output, since OpenFirmware only talks over the serial line. Then I finally found floppy images that would boot actually boot. They were from Woody, so I did a netinst using boot-floppies.
Unfortunately the Linux didn't come up after reboot. After resetting the nvram I could at least boot MacOS again and then start Linux via BootX. It took quite some fiddling with quik and the nvram till I had the direct boot working.
Subsequently I upgraded to etch, but I couldn't get a 2.6 kernel to boot. First I got a bus error from the IMS TwinTurbo graphics card driver, then the kernel "forgot" where the initramfs was loaded, which turned out to be a grave bug in the Debian kernel images (#366620).
I wrote patches for both bugs and now I finally have Etch with Linux 2.6 working. Whee!
The only sad thing is that all this took the better part of last week :-/