A rim with a special track would limit you to that one rim. Not quite as limiting as being restricted to one particular tyre, since you wouldn't need to replace it, but nevertheless a restriction. And most of all, it would require that special rim to be made! Plus, it's not really a problem that needs solving; the drag from a dynamo hub is so small you won't notice it while riding and disconnection is easy enough, very easy with a lego-block connector as used by SP and Shimano.