If the pull of the tethering line disappears - fire
Can't you just measure the weight of the truss + the plane maybe once every tenth of a second. If that weight is less than say 10% of the actual weight it would mean that the balloon has burst and the assembly is falling. If the weight stays below 10% for three (or more) consecutive measurements it's probably not just a wind gust but an actual balloon burst. Use the altimeter to only enable this system above most of the weather to ignore a bumpy start.
If that is not enough use the above to prime the system and then wait for the jerk in the tether line when the parachute deploys using the same weight measurement system and then fire.
Use a longer tether line from the parachute than the height of the balloon to prevent it from covering the plane...