I believe that full up elevator is what the towpilot will instinctively do in a kite siuation. This device will detect that and hopefully not give any false positives

release logic something like:-
1) GPS groundspeed ~60kmh for for ~5 seconds (so its not an engine test)
2) engine throttle ~80% for ~5 seconds (so its a take-off)
3) full up elevator (for more then ~0.2 second to filter out a knock)

when 1) and 2) are satisfied the system is "armed". A low groundspeed or Landing flaps could disarm it

I have chosen a low groundspeed in 1) to accomodate a strong head wind

GPS groundspeed and full elevator are easy to measure. Throttle a little trickier - maybe one could use flaps instead. tweak as necessary

Manual override, arm and disarm switches in the cockpit of course. some LEDs.

a simple embedded CPU costing 1$ can easily handle this and is very reliable. The BOM could be under 50$. This can be built by a hobbyist.

The tricky bit is
* Mounting the sensors
* triggering the release if its not electical

Peter