Its a nice idea. It should be more accurate than a simple sensor/magnet, as it can measure multiple times for each rotation of the wheel.
Presumably it would depend on the number of poles in the dynamo, so need to be calibrated for a particular model.
Would be handy if it could transmit as ANT+, so you can connect it to your usual cycle computer/GPS.
But would probably be a lot more complicated and expensive than a simple sensor/magnet, or an accelerometer style on the hub. So not sure if it would be worth it.