Greetings fellow computer preservationists!
'rfka01' got me interested in this and I am attempting to come up to speed on the rainbow project. I've done some minor simulation work of PDP11 systems in the past and am amazed at the scope of MESS. It will be a while before I can make any useful contributions, but I will watch with interest.
I haphazardly maintain http://www.willsworks.net/pdp11/decmicro.htm.
So I am a Rainbow fan. I would also like to submit DEC's VT-180 (aka Robin) computer for consideration. It is described on the page above with links to the Technical Reference Manual and some disk images which are available at http://bitsavers.org/pdf/dec/terminal/vt180/
This was a Z80 based CPM 2.2 system with 64Kb of memory which was housed in a VT-100 terminal. I think its similar to the Z80 half (CPM80) of a Rainbow.
I've also located some ROM images at http://www.dunnington.u-net.com/public/DECROMs/
per the file, ROMlist, I believe 23-017E3 and 23-021E3 to be the boot ROMs. The character display was handled by the VT-100 board, and I suspect that 23-018E2 will work as a character generator, but a quick inspection shows it also includes some of the setup strings.
I own a couple of both these systems, although only the rainbow is physically accessible at the moment.