If I understand the process correctly, you can't really boot from a divice 
that isn't supported by the kernel itself - deviced supported through modules 
aren't bootable, because the kernel has to have the filesystem open to read 
the modules.  Therefore you'd have to compile the USB and USB/Filesystem 
modules into the kernel for it to be bootable.

As far as getting grub or lilo to read a USB device for the boot source, I'm 
not sure that that would work unless, as has been suggested, the MB's BIOS 
offered USB as a bootable device.

Hacking ROMs and other devices is really beyond the scope of the original 
question.