Out of curiosity, has anyone considered adding the speedometer "sensor" from a Celica? The transmission on the Elise has a hole that would normally have the speed sensor installed - on our cars, it's plugged with a bolt since the Elise doesn't use it.
I would assume that the Toyota sensor either generates a voltage proportional to the speed, or is a pulse generator. In either case, couldn't the speed sensor be added to the transmission and then wired up to the radio, navigation system, etc.?
Edit: I just checked the Celica manual, and the speed sensor is a pulse generator - 4 pulses per revolution of the speed sensor shaft. From the Celica manual: