hacking a computer touchpad to return 2 resistances?

Definitely not just a couple of wires. A touchpad uses a capacitive grid. The onboard electronics sense that and turn it into a serialized data stream.

There is no easy hack to do it. Maybe if you reverse engineered enough, taking the IC datasheet but IIRC these are proprietary ICs and those datasheets may not be available. You can certainly look around for some, one making is Alps.

It won't be easy. Those devices key off capacitive rather than resistive effects so it isn't a case of ripping circuitry out and wiring up to the appropriate points. The best way I can think off would be to get a model with PS/2 output. The PS/2 mouse protocol is relatively simple and well documented. Decoding the signal would be a simple matter for e.g. a PIC microcontroller.

Are you absolutely sure you need resitances though? PWM output or analog voltages would be far easier. What are you hooking it up to? Coming up with a voltage-controlled resistor that works as required can easily end up as a project in itself.

Your first step, is finding out the data format coming out on the PS/2 connector. Convert the output byte codes for the X and Y values, and store them in parallel registers. Connect the registers to digital to analog converters. The output at this point is a voltage. The voltage may be suitable for driving something directly (with a little buffering, say).

Digikey has more than 5000 product listings for DACs, so you have plenty to choose from. Here is a DAC chosen purely at random.

I think a joystick with potentiometers on the X and Y axis, is a *lot* less work...

Paul