What about the temperatures at high altitudes? Would the nicrome wire heat up enough at the cold temperatures at the top?
What I'm thinking right now on the spot is that use a nut and bolt and a small DC motor
Solder/weld bolt on the top of the dc motor, solder/weld a piece of wire on the bottom of the nut and get the nut on the thread with just a few turns... put the nut between two pieces of plastic that form an U shape so that when you start the motor anti-clockwise, the nut won't spin with the bold (but there should be some leeway for the nut to move sideways.
When the thread spins counter clockwise, the nut is locked in place and can't rotate, but as the thread spins, the nut will go out, will be pushed out from the thread and then just flow up with the balloon.
Not sure if there's going to be problems with the metal contracting with the colder air temperature... imho both the nut and the bolt should contract a bit but I don't think