It sounds like boot.img is booted, but it doesn't find any bootable device/image to boot the actual system from. What kind of command line did you use to boot it?
Have you read this guide? http://www.morphos-team.net/guide/usb-boot
I have download the ISO file for Mac Mini G4 (the first choice in the download page) and i hove download 2 times becauise first time i think the ISO is broken.
I have try with the usb boot guide on the page from MorphOS Team.
And i try with 2 usb keys (one 4 Go and one 32 Go).
I have just try with the superdrive of my Macbook Pro connected in SATA but i don't see this in the boot menu from Mac Mini and i don't see usb key in the same menu.
I have download the ISO file for Mac Mini G4 (the first choice in the download page) and i hove download 2 times becauise first time i think the ISO is broken.
Did you check the md5sum for the downloaded ISO image?
And are you sure that you have correctly written the "bi umsd0:morphos.iso" part so that it really matches with the iso image name you have on the USB key (case sensitive etc)?