Yesterday I forgot to turn off ProPadal before quite nasty downhill ride on my Fox RP2 rear shock (riding Giant Reign 2011) - I can imagine this makes no real harm to the shock but think there can be some extra wear of rubber sealing etc? The downhill was about 1km of a very old road paved with big round sandstone rocks (about 0.5 m in diameter), I think I could have used about 75% on my suspension travel, the shock could had bumped about 300-500 times.
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.
I would not worry about it. I run my shocks with ProPedal on most of the time, and something else has always broken well before the rear shock gave out. You're not going to do any extra harm to the shock just because you ran it down a descent with ProPedal on. You should be more worried about bottoming the shock out or not keeping it properly serviced.