I mean the basic physics are pretty simple on a bike -- the smallest surface area that you can for a bike and rider...We've already got saddle to bar drop to lessen the surface area...Laying down on your stomach or back would result in less surface area, especially if the rider was in line with the front tire, and give a lower center of gravity as well...
But I think there has to be a trade off (even in full out racing) between what's sustainable, comfortably and what outputs power efficiently in as aerodynamic a form as possible.
Not sure there's really anywhere else to go when it comes to powering two wheels down a road with the power of a human.