Although I am not an expert on these matters, I can't see why it wouldn't be possible. AFAIK, the BIOS looks for a Stage 1 bootloader to fire off a corresponding Stage 2 bootloader. If you can get the BIOS to find the Stage 1 on your keychain, and you can fit GRUB (or LILO)'s Stage 2 on there, as well as other OSes... well, I can't see why it would be fundamentally different to dual booting from a normal hard drive.
As for actually how
to do it, I'm not your man.