I'm not as prolific, nor dedicated as my internet friend rfka01!
However I did just log in and see he pointed his finger at me last month regarding Rainbow Keyboards. I have never opened one up, but do have some spares I will be near next month if that is required. However I think emulation would be the way to go and the link below gives a lot of information:

In overview the LK-201 keyboard used RS-232 at 4800 baud. I
believe his interface was used with most of the Decmates, the Pro-350, and some of the Microvax which is why its on the netbsd site. On the Rainbow, Decmate, and Pro-350 the 4 pin LK-201 connector plugged into the back of the VR-201 monitor and was then routed to the DB-15 monitor connection on the back of the Rainbow. The Update site below has a lot of good old rainbow info, the file indicated below gives pinout info for this DB-15.

So in terms of hardware, I believe the keyboard acts like a serial line unit on the appropriate pins off this DB-15. On the Microvax the keyboard plugged directly into the CPU unit. Its off topic, but for the record I have a microvax that I occasionally play with using my PC as a serial terminal for the keyboard.

I have always used the MSDOS bios interface to interact with the Rainbow keyboard when writing code for it, and am not sure where that fits in. Presumably the keyboard Uart interacts with the bios routines but I have no knowledge of this level. If useful I can document the key codes produced by the bios routines on my rainbows, let me know.
But I fear we need to obtain listings or disassemble the bios a little to see how it interacts with the keyboard to do an emulation.

I'm interested in this entire project, but primarily in the Graphics option card with which I have been experimenting. I have a copy of a manual similar to the one rfka01 made available a few weeks ago in postscript format, but had never seen the *.DOC version (although they seem identical). For those interested in documentation, if you haven't seen it there is an on-line version of the NEC-7220 manual available, although most of this information is duplicated in the link rfka01 posted, the additional DEC ports and differences regarding the Rainbow are of significant interest, as are the graphics pixels apparently available in the bitmap which can't be displayed!