IIRC the only ROM completely missing which is necessary for accurate emulation from a Rainbow 100B is the z80 arbitrator PROM 23-090B1, which is an MMI6308 256x8 Open Collector PROM at location E13 (E11 on the Rainbow 100A, both share the same part).
I have the 100B. Years ago, I bought a 100A used, but along the way I parted with it. I don't recall now when or why or how, but I think it was at least 15 years ago. So I can't help there.
I found the "Shared Memory Arbitrator" on the sheets, and located the part. I was thinking that I could pop it out and put it in a reader, but no such luck. I can see why you don't have this part. It is soldered into place and I'm loathe to take it out of the circuit. The circuit around it is also a bit complicated, which may frustrate efforts to read it in sutu. Mine is tagged with "6308-1J P8346 \\ LM8438 090B1" if that matters.
This is the first time in years I've looked at the inside of my unit. It's clean and looks good. So the next step is to power it up when I can get a few minutes and round up the right cables... That will let me read the disks I have. The RX-50 will connect to a PC floppy controller, and I have a couple of machines that still have that in them if I can't get this to power up for some reason... I'm taking it slow turning it on after so long since the last time...
The stickers over all the EEPROMs on the boards are blank. I didn't write down the part numbers, though. I should have taken pics while I had it apart to examine later. If I take it apart again, I'll do that.