Actually, the SD card/USB hybrid image should be bootable directly on a G5 if you change the boot-device to UD: instead of HD: in OF and you have written the RAW image to a USBstick directly...
I wished Apple's USB boot device support had been a bit more straight forward back then but they decided to focus on Firewire for marketing reasons, it seems.
Either way, I use this kind of boot method on all my Apple-type machines as a type of emergency boot option, so it definitely works. Of course it can also be used to update/install the latest MorphOS version.
If it doesn't work/boot, it is most likely the type of USB device that is refused by OpenFirmware and/or the image that is not correctly written to the USB media.