Good news, if you are handy someone did make a PS/2 to LK201 keyboard adapter using an Arduino Nano:
Bad news, though the source code is available, the schematics were lost, so it may take reading the source to determine what needs to connect where.... (I checked archive.org and the .pdf of the schematic wasn't archived.)
I recently accquired a DEC VT220 video terminal which was fully functional asides from a little dirt on its case, however, it was not usable yet as I did have the proper keyboard for the terminal.
pbx.sh
Double bad news, it appears not to work well on the Rainbow, but some better news, the picture in this thread shows the UART hat connected to PS/2 and LK201 cables....
However, for 110 or 120 pounds, this company sells PS/2 or USB to LK201 adapters:
Somewhat good news, the LK401 is compatible with the LK201, so if you can find a good LK401... but it seems the LK401 was also known to be unreliable, so any used keyboard is a crapshoot unless it's tested/good.
Absolute best news: There is a newer LK201 emulator project! Confirmed to work with the Rainbow...
Emulate DEC LK201 keyboard using a standard PC USB keyboard - pkoning2/lk201emu
github.com
And it uses standard USB keyboards.
Edit: Did some checking, the PCB is $15 each from OSHpark, and the BOM including the Adafruit Trinket is also only $15, for about $31 dollars plus shipping for materials. Unfortunately, the DIP package version of the RS232 level converter used is on backorder, and so is the 40V NPN transistor. The transistor is probably easy to substitute, the surface mount version of the RS232 level converter is available, but of course would require the board to be redesigned....
Edit2: There are substitutes for the backordered parts available, but they'd add about $3 to the BOM cost.
So $35 plus a micro-USB to USB-A cable (also known as an OTG (on the go) host cable) which is about $6, plus time to assemble and program the Adafruit Trinket.
Edit3: The OSHpark order is 3 PCBs for $15. Tripling the Digikey order with the substitute RS232 level converter is $51, and then another $4 for 20 of the 2n3904 transistors from eBay (cheaper than Digikey), for a nice even total of $70 for 3 units. I'm considering ordering enough to build 3 of these.... with the time needed to assemble and test, I'd probably charge $75 each for them assembled, programmed and tested. To that you'd have to add a telephone handset cable (I may have an e-waste source for those cheap) and the OTG USB adapters (I can get three for $7 on Amazon, so I could include them in the $75 price.) Would take a couple weeks to get all the parts, build and test. Especially since I don't have a VT terminal or VR-201 for the Rainbow I have access to to test... so I'd have to bodge together a 15 pin to phone jack / composite video adapter to test with.