Jeff Armstrong got back to me - he'll dig out his turbow-286 and dump its ROMs.
In the meantime he sent me a real gem - the ROMs for the DEC Rainbow 190, a machine for which the only information on the 'net seems to be "it's rare".https://dl.dropboxusercontent.com/u/55419307/rainbow_rom190.zip
Its secret seems to be (quoting Jeff):
"The Rainbow 190 has _zero_ hardware differences from the 100B. If MESS can emulate the 100B, it just needs the 190's ROMS to emulate it as well. "
"I've attached the ROM dumps for the Rainbow 190. These should work just fine on a Rainbow 100B emulator in theory. They correspond to Rainbow firmware version 5.05A, whereas normal 100Bs use firmware version 5.03A. I'm not sure what's different, and I doubt anyone remembers anymore. I had some trouble with the 190's power supply, so I think I'll wait until tomorrow to figure out which Rainbow has the Turbow card (I have quite a few Rainbow's) and to subsequently dump the ROMs. I don't usually use the Turbow because I suspect it is providing too much load on the aging motherboards it's attached to.
I'll also be posting the ROM dumps to rainbow-100.com when I get around to it."