sir GT, the problem with your idea (that i see/or think might be a problem) is that the axle is covered. and if you uncovered it to use the brake idea it would be pointless almost. teams would rather not carry the extra weight. and why would you want the mini-pads to clamp down close to the axle? there would be more tq applied to the pad there. if anything you would want them to the farthest away fromo the center of the axle so you could actually multiply the leverage of the brakes. i think thats what you were saying, its kinda unclear.
also, we discussed this subject a wile back over at a car board i'm part of. anyways the only logic i can think of is that in racing if you had brakes like this you would have to have to get new wheels to and such to fit them on (complete re-structuring of the wheel) and even then, you would have to use a bunch on small pistons which might not me as effective as the larger ones.
wheres that dude that works for delphi, maybe he can enlighten us