I was surprised to learn that the Payload droppers where a touchy subject with the F.A.A. Companies sell them, and I have a couple as do a lot of people. BUT as far as if they are actually legal can open up a whole debate at a F.A.A. safety meeting.
That wouldn't surprise me at all, given the way drones have made news with payload dropping capabilities in recent times.
Even a simple payload release might help the OP, if they can do it legally.
A small soft ziplock type bag with paint, again just a one drop system, maybe to test their plan.