I've flown on a number of helicopters and every pilot I have come across are very cautious about anything that has the possibility of coming loose and hitting the rotor. I would be beyond shocked if a pilot would allow a passenger to attach anything to the outside of the copter. Even when I've done photography with the doors off or windows open they have been very strict about even allowing lens caps to be loose on the inside of the craft. I don't think I'd want to fly with an operator that allowed it...