When I tried this a month ago, on an iMac G5, I found that the 12.10 iso and my graphics driver wouldn't let me get to the try/live desktop - or if it did, with no hardware acceleration, the colours on screen were posterised and practically unusable. I found two solutions, one of which you'll probably like, one you won't:
First, (and this worked for me) you could try downloading the alternate PPC install iso and running from that. It allows you to install using the non-graphical installer, with a debian/DOS-like interface. Once installed, you should be able to boot up into the OS, but will need to tweak any parts that don't function correctly. In my case sound, wifi and graphics. The solutions are out there and are google-able. I settled with a 98% perfect install, just having to disable the hardware acceleration in yaboot every time I boot. For a glorified word processor and email client that's fine, but for youtube, DVDs and flash its game over.
Second, you could try downloading the 12.04 version and running the older installer. That one worked fine for me using the graphical try/live desktop. Once installed, you could try updating the whole operating system to 12.10 if you wish, using "sudo do-release-upgrade" but a word of caution: when I tried using this technique, the nouveau graphic package got corrupted, so I had to start from scratch.
