It is possible to set the boot device to whatever you would like, so long as the ROM image and CMOS image get loaded by the firmware, and you have a FileCore device to boot from.
Though remember that RISC OS RPi is hard-coded to save the CMOS setings to the SDCard, and you would be wasting one device doing this. Because the magic trick that RISC OS uses to put its filecore filesystem on the same device as the FAT32 boot partition is not easy to repeat, and RISC OS does not support partitions.
So as an alternative I would recomend just loading the RPi Firmware, ROM, and CMOS settings from a 64MB micro sd card, and having the on disk part of the OS on a USB drive, the ROM will boot the USB device once you have everything configured correctly. This is pretty much what I do.
RPi = Way for me to have fun and save power.
100% Off Grid.
Household TTL Electricity Usage = 1.4KW/h per day.
500W Solar System, produces 2.8KW/h per day average.