For chucking text files back and forth I have had relatively OK luck just "cat"ing a file to or from the serial port, with the caveat that XON/XOFF flow control and a low baud rate make it more reliable.
This works relatively well for logging in to a getty, too. I haven't spent enough time poking around launchd to tell you how to fire up a getty on Darwin but it ought to be possible without too much hassle and you could always just run one by hand for testing it out. There's a great terminfo file out there that pretty much Just Works; I think it was b 9's efforts to get it into such nice shape. Because of the variable processing speed of the BASIC interpreter, just blasting away at the serial port with cat rarely works great when I try to do it to LOAD untokenized BASIC. My best results by far have been with using dlplus bootstrapper mode. It's been decades since I used minicom but I think it does offer the ability to enable software flow control if you don't want to wade into the intricacies of stty.
