It seems like the easiest and safest route would be to get a hold of one of these places that specializes in fire systems for race cars. There is usually one in every major city. They build the auto discharge systems for almost all race cars. One of these with an alarm to let you know when it is going off would be nice. I would guess it would be spendy. But maybe worth it.
It would be interesting to know what these systems use.
hope this helps,
Craig
the Gorge VW club