I'd have it keep the instance live and running while you're DC'd. The timer keeps ticking and enemies keep spawning (and patrolling), but if you reconnect and aren't dead when you get back, you can keep going without losing the entire run. (Of course, if something spawns on top of you and murders you while you're reconnecting... too bad.) Only way I can see them at least partially mitigating this without leaving it open to abuse.