Well, if you can get quadrature output, or a analog voltage, you could use a PIC and an
SPI digi-pot. The SPI protocol is simple enough to bit-bang, and there is 8-bits of resolution, which lines up nicely with the OI.
Of course with that, you could start getting into look-up tables allready on-board the joystick, on-board mixing, and all kinds of fancy stuff... Got a kiwi drive? use the PIC to mix your 2 joysticks, and send the desired motor values to the RC, which can still use PID or whatver kind of control you use.
I may develop a simple PIC based converter this summer, if I do I'll make the source code available for use, and sell pre-programmed pics with the code on it.
Though if you want to get fancy, it might be neat to use the force-feedback on a sidewinder for something. Maybe a stick shaker or somthing if the CG is in a bad position, (robot about to tip...) or make it more stiff going forward if it detects that the wheels are slipping, (I.E. you are shoving something)