FAA and other CAAs seem to have no interest in the rope. So a KGARS could be installed between the towplane weak link and the main rope.

You would need angle sensors (as available for digital levels) on the towplane elevator and stabiliser to send a flash to the KGARS unit when release parameters are satisfied.

That would be when the stabiliser is pointing downhill while the elevator is at a steeper angle.

There are remote photoflash units that trigger on a flash from a master unit.

So you need a KGARS that on receiving the flash would let go as long as it's not from a Whelen strobe.

Alternatively Bluetooth can transmit over that distance. A stick switch could also signal via Bluetooth, but might need the transmitter at the tail.