If you can, please dump the ROMs of your Rainbow 100 A ... looks like you have quite an early set from the boot message you posted.
I've dumped it and uploaded it to rb-100a-1.1.11a.bin
but I've not tried to load it into MAME yet.
It's 24k. I used debug to copy FA00:0-FA00:5FFF to low memory and wrote it out verbatim. It has the sorts strings and instructions I'd expect in it to have when I disassemble it that are similar to the other ROMs that are available at least at first blush. while I do have a ROM dumper, I've never used it and am hesitant to remove the physical ROMs just to read them if other methods can extract the same data...