So I came up with this and finished it today. It cost $10 and wasn't really hard to build. Best of all, it actually works as intended!
It consists of a 2" plug and a 2" x 1/2" bushing glued into a coupling.
A short length of 1/2" pipe runs through the bushing to the inside of the chamber and is capped. The cap has a pin-hole through it. The pipe extends out of the large bushing into a 1/2" x 3/4" bushing which connects to a 3/4" union.
The idea is simple. You fill the mine through the schrader valve and it fills up. Slowly, air bleeds into the small 1/2" chamber until it ruptures a burst disc, thus firing some baby powder or whatever you put in it.
It does work, but I'm still experimenting with burst discs to find the balance between power and an appropriate delay.



(more pics and hopefully a video soon)