Mechanical Trigger Failsafe
I was thinking of utilising the fact that there'll be tension in the chords between the truss and the balloon just before it bursts and none just after. I was also thinking about those spring loaded tape measures that snap back when you pull out the measuring ribon. You could have some kind of spring loaded system such that there's a chord connecting the truss to the balloon at tension while the balloon is ascending, that will snap back into the truss when the balloon bursts. The end of the chord attached to the balloon could form an electricital contact for the trigger that will make contact with the other... um... contact when the chord retracts back into the truss.