​Actually there was tool that was almost all in userland to support multiple sessions over single serial line between a Macs a UNIX system.  My memory is that it used Chesson's multiplexer (mpx) which is part of stock V7 (his is pre-select system call).​  I wish I could remember the name of that program.  But I bet it or something like it could be repurposed pretty quickly to talk to a frontend micro.

Biggest issue is interrupt overhead on serial ports on the 11.   If this is real HW, see you can find a real DEC DH or better yet - an Able DH/DM.  DH style interfaces will be a huge difference over DL's or DZs.  DZs were pigs on Vaxen and on an 11 a line at 19.2K continuous could kill it.

BTW:  I thought of another option.  It's not telnet or ftp, but if your desire is move files back and forth without having to use a common physical media and sneaker-net, BSD 2x should have the BerkNET code in it.   That will run on an serial line - although my previous comment about the type of interface can matter from a performance standpoint.

Clem