@Henry crun
Not a pilot, just an avionics guy, so tell me to go away if you wish.
I would suggest the use of totally separate, lane segregated, battery powered gps at the four corners of the airplane. Collectively they could report a dangerous rate of descent. Differentially they could detect inappropriate attitudes.
Gps accuracy is not good in airplane control terms, but it is definitely accurate enough to sense outrageous errors.
Why not?
Interesting idea.
In fact GPS is amazingly accurate when used differentially to calculate attitude down to sub mm accuracy. The timing pulses in the GPS satellite data packets are used to generate relative attitude information instead of the spacial reference data. This has been demonstrated successfully in many small civilian aviation devices.
The problem here is that the wings have tremendous flex especially at the ends. Even the fuselage has a significant movement at the ends. This would have to be compensated for - maybe with laser level.