Hmmm ... I'm missing something here:
I compiled MESS, put the BIOS and chargen images into rainbow.zip, put it in \roms, put the disk images mentioned in rainbow.xml into roms\rainbow, but MESS won't let me start emulation, it comes up with "The selected game is missing one or more ROM or CHD images".
This procedure was the result of some searching, so please be gentle with a noob ;-)
Robert
