Thank you both for your help. Jaca helped me further on IRC.

I ended up managing to boot with the following command:
" boot usb0/disk@1:,"(5C)boot.img bootimage=umsd0:morphos-3.1.iso" evaluate

The reason for "(5C) and evaluate is that my keyboard was not willing to produce a backslash in any other way.

I am typing this from OWB and now I just need to get it registered and pimped. Thanks again! :D