Thanks for the link, wheagy. Good info in that thread, looks like I'll try Minicom for the xfers.
I have used FTDI chip USB-serial adapters and also GPIO serial with the level converters on the Pi with success. There are many Pi config options to choose from to customize the way you access the Rpi and the way you access devices from the Rpi. For example, you could run a clone-connected Rpi headless and ssh into it over the network, then access the clone from there. That way, you wouldn't even need a keyboard and monitor dedicated to the clone.
All in all, I think the Rpi and the Altair clone go well together. I'm currently using a Pi Zero W with the clone and it's really tiny. But very capable as a serial server. Not as powerful as the Pi 3 but you don't really need it to be, if all you're doing is serial/CLI.
I like the idea of putting a Rpi inside the clone. May do that eventually.
Awhile back there was something called Pocketerm
http://www.brielcomputers.com/wordpress/?cat=25but it is no longer available.
This seems like a similar project:
http://geoffg.net/terminal.html