Well, the best part is, the computer accepts no inputs, so it's all reading, no writing
It just takes a 10K resistor on the "mode" pin to make the computer start rambling 160-baud data, in a simplex stream, then it's up to the receiver to figure out what the gibberish says. That's why I think it wouldn't be too difficult to implement, since it's basically just another timing-and-computation sort of thing, similar to reading the VSS.
Thanks for the tip on the zener, though, that'll definitely help simplify things, I wasn't sure how the system would interpret
approximately zero volts for a "low" signal, but a zener sounds like just the trick.