I'm not a Rainbow specialist, but I know there's "DECnet DOS" (e.g. here). As I'm experienced with VAX/VMS I know that DECnet over serial lines was rather common from VMS V4.0 onwards, sometimes called "Poor Man's DECnet", because you do not need an expensive Ethernet Adapter. So if there's a DECnet DOS suitable for the Rainbow (or another x86 DECnet software) and if it supports serial DECnet, you're fine!Decnet was more common for the Rainbow and there were some methods of transferring Decnet over to Ethernet. A solution may already exist.
I would not describe the Rainbow as used in great numbers.
Thank you for that. It is very helpful.DECnet Rainbow seems to be available here:
https://oldcomputers.dyndns.org/public/pub/mirror/os2site/sw/dec/rainbow/msdos/decnet/index.html
And I found a helpful manual (more than the title promises) in my collection: