Well I played around with it last night. The spring tension is so strong that a bungie cord would have to be pulled very tight to keep the discharge chute deflector in the up position.
A wedge of wood, on the other hand, looks like a good answer if it doesn't get knocked out. It should also be pretty friendly to the paint finish and would easily fit inside the toolbox so I'd always have it handy. I'm thinking of making up 2 blocks just in case? Or maybe drilling a hole in the block to attach a string to it with a clip on the other end of the string, that way if it gets knocked out of position the string would keep it from falling away from the tractor and getting lost in the dirt, etc.