re: VSS speed sensor.

any box that can run Linux can easily do this - 25 pulses a second is nothing.

But - the problem you have (with the Empeg in particular) is that there are no spare input pins to the Empeg, the 2 that the serial interface has are multiplexed for phone and display dim {headlight sense).

So, you could use one of them, but you'd have to sacrifice one of these 2 features.

For Rallying, I suspect the Cellphone mute is probably the one to be used here - heck who wants incoming phone calls when trying to stay on the road right?

Its then a simple Kernel mod to "count pulses" and then store this information somewhere accessible [e.g. /proc/empeg_XXXXX file].

Second issue then is having a program to show this information.

For a regular Linux OS [other than Empeg], then any regular serial port will do the trick, but again you'd have to write some software to do the polling and display.

But using a non windows OS for this software/hardware could be done.
Its a matter of someone doing the programming.