Hey, I just found something. It seems that someone solved the "chicken or egg" problem with Portfolio's serial interface: the computer has no serial communications software built in, so it's impossible to send any from the PC and, unless you have a parallel interface, you're virtually unable to use the serial port to transfer files. Digging through the files on atari-portfolio.co.uk I accidentally spotted this. It's a tiny program that sends Xterm2 to the Portfolio over the serial port.
I haven't tested it (for I don't have a serial interface), but I bet it would work, with a simple yet brilliant idea behind it.