I wondered about RTTY programs. It turns out there
there is one, called
"HamComm" that can do 5-level code at 50 baud. Best, it can runs on a PC
XT. See
http://www.pervisell.com/ham/hc1.htm
Note the option (23) in the README file for "external converter", using
the 232C handshake line for data input/output.
Maybe just the ticket.