[TUHS] Stdin Redirect in Cu History/Alternatives?

Good morning all. I've been doing some historical research on the UUCP cu utility this morning and have come across a little discrepancy between the various UNIX streams I was wondering if someone could illuminate. So cu as of V7 supported the ~$ escape, a means of calling a local procedure and emitting stdout over the TTY line to the remote machine, all fine and good for packaging a character stream to emit. However, what I'm not finding in that age of documentation is any means of requesting std*in* from the TTY line as input to a local procedure (in essence running a text filter or handshake-driven protocols over cu). The context in which I'm researching this is integrating cu into my bare-metal SBC programming using XMODEM so I can rest a little easier my process is based on tools I'll probably find in most places. So old fashioned Mike Lesk-era cu only seems to do stdout redirect, but no stdin. I did some further digging and it looks like different UUCP implementations cracked this nut with different escapes, with BSD eventually going with ~C and Taylor UUCP opting for ~+. Checking the current illumos manual pages (for a SVR4-ish example) doesn't turn up any command for this. This is indicative of there never being an agreed-upon mechanism for doing this, although I could see this being a very useful mechanism. What I'm curious about is if the lack of a bi-directional redirect in early cu is reflective of a lack of need for that sort of functionality at the time or that matters such as that were handled through a different mechanism. One thought I did have is that there wasn't file locking at the time (right?) and so theoretically nothing would prevent one from using a cu session on one terminal to send interactive commands and then a second using fd redirects in the shell to run filters/protocols separately of the interactive stream. - Matt G.