If I did something like this I see myself sticking with whatever gear I started out in for the most part. For example, when I go out my front door, I have two choices--one way is straight up into the mountains, and the other direction is pretty flat. Right now my singlespeed is geared pretty low because of all the potential climbing. Could be cool to be able to swap my "singlespeed" gearing on the fly if I'm gonna head into town and stay in the flats all day. I can always just take my geared bike of course, but this idea seems nifty.