the answer is... maybe

I was able to start my 12" off a USB DVD burner I had, but I was unable to boot off a 32GB Lexar USB 3.0 thumb drive that I imaged 10.5.8 on

the DVD I just held the Option key and selected it... the USB you have to do Option+Open Apple+O+F to get into open firmware and then try to boot from the device "ud" or usb disk (I guess?)

more info here

https://sites.google.com/site/shawnhcorey/howto-boot-apple-powerpcs-from-a-usb-drive-in-open-firmware