You have to insert the stick *before* booting - otherwise OF won't recognize it. And it should be a smaler one (>4GB). Try different sticks - MACs a a little bit picky.....
Some OF-Versions recognizes the stick, but doesn't create ud. In this case , there is a way to access the stick with it's hardware-address.
Till yet I have'nt found a G4 or G5 MAC, which can't be made to boot from USB.

FYI:
1. Partition (HFS) need not to be made "bootable" only "mountable". From this partition you load boot.img directly with OF-command above.
2. Parition ist SFS an contains MOS-System - this partition has to be set bootable, so that MOS-Kernel can find and boot from it.


I've done a manual, but in German (https://www.amigaportal.de/index.php?forums/morphos.47/). There are some Tutorials for alternate boot.devices and Multiboot-Systems. Maybe you can read German or translate it by Google.
If I find time, I can do a short abstract for your case in english.....or someone translates this tutorials professionally.