If your using the head part of the fish just bring some rope thread it through the mouth and the gill then hold on to both ends of the rope and just give it more line until it reach the water then release one end of the ends and pull it out.
You might try to use a "sling" of some kind. Maybe a hammock (solid not net style) rigged up with rope long enough to go all the way down to the water. get your bait lowered to where you want it and then let go of one end of the rope. The "sling" should open and out swims your tarpon. then just pull it back up. this would also work for releasing/landing big fish without using a gaff. Just an idea
do as said before, pay out enough line off the bridge to hit the water, now you have twice the length you need and chuck it off the side. the balloon wont break off and you can replace the balloon with a 1 gallon milk jug. #15 wire looped around the handle and attached with a snap swivel to a swivel on the leader. toss a glow stick in there and you can see it at night.