I would think that a 'dead man switch' would be appropriate. Can you just dedicate one channel to a continuous broadcast that orders a relay to remain open? When the signal is lost, the relay should energize and trigger your 'chute system.
edit: I guess that I mean 'de-energize'. I was thinking of the secondary rather than primary circuit.
edit #2: I just noticed that you also have this posted in 'Mechanical Engineering', and Topher gave you a great response.