Here's an idea - build a fixed gear hub with a little clutch that is disengaged by a lever on the handlebars. When you get to the top of that big hill (for all you San Francisco people, imagine the Headlands loop or Mt. Tam) you pull the lever, disengaging the clutch, which allows you to coast down the hill. The lever would have an indent or some method to hold it in position until you are ready to re-engage it. You could then slowly move it back into position. The cranks would start to turn but there would be slippage to allow your legs to catch up before the bike is completely back to fixed gear mode. The one possible drawback I see is that if the clutch were to suddenly re-engage (broken cable, lever pops back out) then you would get tossed off the bike and into the ocean where the great white sharks would munch on your carcass and crabs would eat your eyeballs. (Or if not doing this on a coastal road, substitue vultures and rats for sharks and crabs).

But it does sound kinda cool, right?