Here we go!
I tested it out by screwing the lid on tight and immersing it in a bucket of water. Even without plumber's tape on the threads, no air escaped. I might do the pressure tests that way, with the thing in a bucket. That way, if it does blow somehow, the water will reduce the velocity of the shrapnel. I still don't see it exploding. ABS pipe is really thick and the inflator stem is affixed really well.
Still, I haven't had the courage to try inflating it too far yet.
Background info here.