Tom Jennings writes:

Using serial ports is probably not a good idea on PC clones. The IBM hardware design limits a machine to two ports comfortably, four practical maximum. Any machine that uses telecomm. (BBSs, etc) will have to consume the serial hardware, and will interfere with whatever you have to do.

A good choice would be a parallel port, ie. a printer port. The IBM design allows 4 or more easily, and rarely do you see a pclone with more than two printers attached. If there isn't a spare port available, Fry's sells printer adapters for $19.95. Late-model printer adapters are 8 bit in and out, and are capable of Ethernet speeds. The parallel port uses standard busy/done and TTL levels.

I already thought of that, but it would make it difficult to work on other platforms. Tom also writes:

A simple driver could simply have say 10,000 bits, and continuously overwrite the oldest (wrap around the buffer...)

This was (and probably still is) my overall intention.