if the PowerMac isn't able to read a FAT32 formatted USB stick, then things are going to be a bit tricky I guess :/ It's good that you can read the boot.img from HFS+ - at least I know that you'll be able to boot from USB. The problem you are facing is that support for HFS+ on MorphOS is in the ISO itself and not the boot.img - the boot.img therefore cannot boot from your USB stick. What you need to do is to format the USB stick in HFS (not HFS+ !). It should be relatively easy to do from Linux or from a PPC Mac OS X.
