What keyboard are you using to type those OF commands? If it's non-US, you may be using a wrong character instead of the backslash.

See https://morphos-team.net/guide/usb-boot#section4 'There is no backslash on my keyboard. How do I type it?' for an explanation and hits on how to overcome this.

'boot ud:,\boot.img bi umsd0:morphos.iso' should definitely work on your PowerMac, unless it's unable to detect the drive at all. Make sure the drive is <=32GB since OpenFirmware has problems with larger FAT32 formatted drives. Use the USB ports at the back, not the one at the front for added certainty.