The laser can give you altitude to within millimeters. Scientists have used lasers to determine the distance to the moon, and most common rangefinders use lasers these days. This is one the most accurate ways known to determine distance. Determining the altitude isn't the hard part, displaying it so the pilot can maintain it is harder, and the hardest part of all is staying at the correct altitude, and over the correct point. A 4-axis flight director, coupled to a suitable electronic device, should be able to do it easily, but the cost won't be cheap. The US Coast Guard's helicopters can do this from low altitudes, but I don't know about the altitudes required by the OP. I think some sort of laser equipment would be the best way to go, if the funds are available, and it will require a rather large pile of them, I suspect.