Re: Mechanical Trigger Failsafe
How about replacing your tape measure with a simple spring scale?
Chose a spring scale with a range of 1/2 to 1/4 (guesstimate) of the total weight of the truss (incl Vulture 2) so that once the balloon's inflated and the truss is suspended the spring scale will be fully extended and at the limit of its travel, so as to avoid bouncing off the upper stop during the ascent.
Botch a new lower limit stop, at about 1/8 (another guesstimate) of the full range, so that the spring is still under some tension when it fully retracts. Then epoxy an insulated contact onto the scale, beside the new lower stop, and another to the spring scale indicator so that when the balloon bursts and the spring scale retracts it makes a firing circuit.
I think the the idea of attaching anything to the parachute canopy is risky because of the possibility of it interfering with the opening of the canopy. As well as the risks of the canopy failing to inflate quickly at high altitude and of the long release cord getting twisted around the tethers, there's also a risk of it being blown taut and into a loop (think of a fishing rod with only a top eye and no intermediates) which may or may not interfere with its intended operation.