Thanks Colani1200, seems we're getting some progress here. I'm now able to transfer files, but they're corrupted when they arrive. Since I don't have the preferences program for the serial port I can't alter the serial settings. That is, I have no idea how to do it outside the prefs prog.
However, I'm pretty sure the default settings are 9600 baud, XON/XOFF handshaking, 8 databits, no parity, 1 stop bit. I've tried both the "type" approach (which apparently doesn't work correctly with binary files from WB2.x and up, the files are chopped off at the end, but text works), and an Arexx script that can be used with ADF Sender Terminal. The latter yields correct file size on arrival, but they're still corrupted.
I've followed the instructions
here carefully. God, I feel like such a newbie again. Lol.
EDIT: I have this nagging feeling I should use RTS/CTS for handshaking rather than XON/XOFF. Is it possible to change this from outside the serial prefs program?