The more I thought about this today, the more refined the idea got.
Someone asked "who's going to pay for the R&D". the answer is , *I* would. If it meant I owned the rights to the product, it would easily make financial sense.
You are right, it would be very difficult to get a device that taps into the rotor and engine rpm signals approved. That's why the box should have a good accelerometer installed with a fast enough processor to do a real time fourier transfer and pick out the engine and rotor RPM from the airframe vibrations. Any mechanical engineering grad student could gin this up, I'm sure. Then, the legal issues are greatly reduced. Just solidly mounting a box.
You don't think most flight schools or lease-back owners would pay $1,000 for such a box? Sure they would. Pays for itself with one overspeed.