So, after a bit of thought.....
I would rule that you get no relief. Where the ball was on the bridge was within the boundries of the hazard. Therefor the ball was in the hazard, thus no free relief. Your options 1) play the ball as it lies 2) take a drop outside the hazard, no nearer the hole, 1 shot penalty 3) go back to the spot of the previous shot and drop with a one shot penalty
If the ball was not within the confines of the hazard and ball bridge was in the line of play.... Free relief, no nearer the hole.
I've got to agree with you, Rick. Following TT's lead, I also looked over the USGA rules. Rule 24-2b states: "Except when the ball is in a water hazard or a lateral water hazard, a player may take relief from interference by an immovable obstruction as follows:" Since I could fin nothing that says the bridge is not in the hazard, the ball on the bridge--even though the bridge itself is an immovable object--is in the hazard and can only be played under the rules relating to relief from a hazard. In either instance, the end result was 7, so it really is moot.
Thanks to everyone for chiming in. It shows the type of odd thing that can happen during a round, official tournament or not, which we as golfers should be knowledgeable of. I'll post another one later for everyone's perusal and amusement.