There is a serial port on the MiST board, BUT:
- it is either inside the case, not available as a connector on the case
- or, it is used by the MIDI ports
If you feel like hacking a little bit, you could add a few wires and make a hole in the board, or alternatively, make a MIDI-to-serial adapter (MIDI is of course connected to the minimig serial port).
Another option is to implement some code to handle a USB serial adapter, AFAIK it is already working on the Atari core.