Does ProPedal on FOX RP2 rear shock wear out when on during downhill?
While shocks are certainly meant to be locked out and ridden, even over semi-rough terrain, there are probably some limits to the design. I'm not a fork/shock mechanic, but what I know about fluid dynamics tells me that you shouldn't do this too often!

When you lock out your fork/shock it limits that compression of the fluids inside or prevents compression at all by preventing other parts from moving.

Scenario 1: The fluid isn't allowed to move through the damper. You could cause high pressure build up on a seal or damper and cause it to blow, or even rupture the canister.

Scenario 2: The parts are prevented from moving. The forces exerted on the locked parts could be more than they were designed for and could cause premature wear to the point of failure.

Again, I'm not a professional, but I wouldn't recommend riding like this regularly. Perhaps a quick email to Fox might be more helpful, as they may have actually done testing to see what happens when a rider rides the rough stuff with a locked-out shock.