I think this is a great idea!

Most cars use a pulsed variable frequency signal from the transmission to drive the engine ECM and speedo. Maybe that could be connected to the mic input? No serious hardware required, if any. Maybe just a resistor network to divide the signal down and a transformer for isolation. Sounds like a relatively easy thing to build into the kernel/Hijack. Since different makes/models have different calibrations for their speedo's, there could be some parms in the config.ini so it could easily be customized. And if it's done in the kernel, I don't think the volume bar would pop-up.

Cheers,
Phil