OK, first of all... I'm pretty sure you're not allowed to be connected to the program port during a match.
Second (and yeah, I am thinking about doing something like this), I'm pretty sure it can be done, and easily. Here's how I would do it:
For inputs to the auxiliary Stamp, feed it with either PWM or relay outputs from the RC. If you want to use PWM outputs, just make them operate digitally, ie. give it either 0 or 255. Then just read the digital data from off the pins on the aux Stamp. You could use one output as a clock to send more data.
To output from the Stamp to the RC, I would say just connect the IO pins of the Stamp to the digital input on the RC. You might have problems with using analog inputs, because I'm not sure how they sample and PWM isn't really a variable analog voltage. You could use a clock line here, too.
If you need more pins on the Stamp, you can try the 40 pin model.
Just a thought, still has a lot of stuff to be worked out.
Eric (an electronics freak)