I'm just guessing here, but I think those micro R/C cars have controls that are just switches (i.e. full left, full right, full forward, full reverse). I think it's the same deal with cheap R/C toys, with the exception of maybe a pot control for the throttle?
So to make it work without having a cable tethered to your computer, and with the least amount of parts, I'd say hack the R/C controller. Use the parallel port and interface that with the controller: transistors or h-bridges for the switches, and a programmable resistor for the pot.
Voila! Seems like an awesome project - good luck!