GNU Screen has a command called
exec that will launch a subprocess and optionally let you do things with its file descriptors, including tying them in your window. I frequently use
:exec !! sx -b /tmp/file to send a file over a serial link using the xmodem protocol.
I'm looking for the same functionality in tmux so I can move away from screen without having to use minicom's (or picocom's, etc,) function for sending files.