Automatic control for a nuVinci?

Hints from Interbike suggest that a controller will soon be available for the nuVinci hub to keep it at a constant cadence. This would be great on the wheelchair bike, as the posture is all wrong and freeing a hand to change gear on the steep dip on our street is very hard. I wonder if it would keep up with the rapid pace of change? I've considered building a simple controller myself, probably based on detecting the chain links interrupting an IR signal, but haven't done anything in that area and lack the time. So one you could buy would be great...