I've seen anti-squealing spring clips that push against the pad edges to reduce rattling/vibrating, but nothing that pushes side-to-side to retract the piston(s) - they could be out there in motorcycle sizes, though. The bicycle guys are fanatics about drag, they might have something that would work.
If you want to reduce pad drag, you might want to try a caliper service. I just did mine at @ 3.5K miles, and the J Juan 25mm rear - which worked poorly and had significant drag - was awful; both sliding pins were bone-dry and one was rusty, and the piston was nearly impossible to move by hand. The Nissin front was much better, with greasy pins & pistons that moved pretty easily, and barely any rotor drag.