Hello,
I didn't find any PPC USB Distros, but I found a site listing Distro's that run on PPC.
http://www.penguinppc.org/about/distributions.php
I'm only slightly familiar with gentoo, I have no experience with the rest. I know you can make gentoo small, so you might be able to build it to fit on a USB drive.
DSL (Damn Small Linux) is built to run on USB, but from what I can see, is not PPC compatabile.
-Jason