I wish I could think of what it was, but isn't there a feature where it tags everything on to the end of the BIOS stored in RAM from an XBE? I think it was some feature in EvoX dash, but I could be wrong, I was reading about it on these forums the other day. People said theoretically, for whatever it was to work, it would tag that code on to the end of the BIOS stored in the memory bank. (and no, I'm not thinking of IGR and XBE's instead of BIOS).
Now, could you not have a small, bare BIOS, that comes with an accompanying "Boot.xbe" that would boot from Boot.XBE, let you chose your BIOS from a menu, extract .BIN's to a Temp folder the way a BIOS would, then replace the BIOS in memory with the extracted BIOS of that folder, then send you to evoxdash.xbe? You would need direct access to the memory banks, but it seems to me it would be the best way to go at it.
Then again I'm just talking out of my ass, its probably way off.