I seem to remember there was a similar problem in WW1 with fighter aircraft shooting their own propellers off. Allegedly, the Germans solve the problem by synchronizing the machine guns with the engine rotation; the English solved it by putting metal reinforcement over the but of the propeller that was getting shot up.
Does that help? Probably not. I suppose you could find out when the wiper passed the park position by reading the state of the park switch. Or by having a light sensor looking through the windscreen that detected when the blade passed over it.
The Germans extended a shaft from the engine, that had cams on it that lined up with the propeller blades. The cam lifted a rod that disengaged the machine gun trigger every time the propeller blade was in the way. It worked pretty well, until the cam wore down, or the rod bent, and the pilot shot their propeller out.
Couldn't this be solved with a limit switch placed on the bottom of the windshield, so you would be able to tell when the wiper was out of the way?
Edit: or a light sensor like you mentioned, would be easier.