Putting fibers in the bra that could be electrically contracted could do this, when combined with a power source and a bounce sensor. You'd put the fibers parallel to the ground and contract them to compress the breasts agains the chest when detecting bouncing. I think the biggest problem is finding a good contractile fiber. Looking, I see what might be suitable is the bio metal helix. You run a current through it, it heats up, it contracts. You turn off the current, it cools down, it relaxes. It contracts more quickly than it relaxes, but that's probably fine for this application. It's hard to see what the price per meter is in bulk, but I suspect its expensive. Maybe in huge quantities it could be made cheaply?