There may have been Rainbow BBS programs, but I doubt anything for the
  11/34.  You may have to write this.
 
 That reminds me of a bit of obscure trivia...
 Back in the early days of FidoNet, one or more of the Fido BBS sysops had
 DEC Rainbows.  The machines could run Fido just fine, but the serial port
 address/port didn't follow the convention laid down by the IBM PC.  At the
 time, there were other MS-DOS compatibles that also had a similar issue
 with the serial port and some of those folks wanted to run Fido.
 Tom Jennings, Wynn Waggoner III(sp?) and Thom Henderson(sp?) got together
 to create the FOSSIL standard.
 FOSSIL is Fido Opus Seadog Serial Interface Layer and provided a mechanism
 via INT 14 for any MS-DOS compatible computer to run any BBS or mailer
 software that had FOSSIL support and a FOSSIL driver available for it.
 FOSSIL continued to be a thing long after the issue of serial port
 incompatibility was a thing of the past.  In fact there's modern software
 out there now such as NetFossil that telnet-enables software that can talk
 to a FOSSIL driver.
 The two popular FOSSIL drivers that I recall from back in the day were BNU
 and Ray Gwinn's X00.
 As an aside, if anyone has or knows where I can find the source code for
 Opus BBS, I'd be interested in hearing from you!
 
 That's what I was thinking.  I have some FidoNET files and mail from the
 Rainbow.  My guess the BBS would have been written in Pascal or C if for
 the Rainbow (guess only) so if you wanted to attempt to port, after you
 find a Rainbow BBS?  I'd start with a Rainbow BBS disassembly/decompile and
 see if you can convert to the PDP 11 running the same language/compile it.