Ok, so I found it in the electronics recycling bin, but it's soooo pretty, complete with original kbd & mouse. And it works too. I couldn't resist trying to put Devuan on it, and much to my amazement the installation zoomed along without error - and it started booting afterwards too. Started, but then it got to fb: switching to radeondrmfb from EFI VGA and just froze. At first I thoght this was a crash, but it isn't; it's the GPU that stops updating the display - I can still ssh in and the system is running. I rebooted, editing grub and adding nomodeset, and voila, a surprisingly snappy desktop - as long as you don't do anything which needs GPU acceleration, like well, almost anything.

CPU performace is not bad; this is a 2GHz Dore 2 Duo with 4Gb RAM (though I suspect only ~3Gb are usable) so not hopelessly... hopeless. I just need to get radeon to survive the modeset and it will make a sexy little DVD player / casual browser. It turns out there is a way to make it go: if I can get the iMac to boot in "BIOS Emulation Mode" then the video BIOS should work with radeon. But how? I tried re-installing a couple of times, manually creating an msdos partition table before partitioning, but the installer kept failing when it came time to install grub, and even if I got past that it would not boot. So I'm hoping I can get some help here - how should I configure the disk partitions during install so that

1) The iMac will boot into "BIOS Emulation Mode"
2) I can complete the installation of a bootloader
3) The machine will find the boot partition afterwards

The patron saint of abandoned technology will be forever grateful for any assistance.