OK, so my mind might be a little juvenile when it comes to this stuff because I was still a juvenile when I thought of a lot of this stuff. And if you're talking disruption, it's kind of juvenile anyway. But anyway...
Isn't there some sort of really bad fart-smelling stuff that could be released? Or even sulfur? Or, with the right beers and other foods, you could make your own. Guaranteed to make a few people uncomfortable and disrupt things. The more 'natural' it smells, the less likely they will be to suspect it's anything but gas.
Like you said, a lot of sound systems are wireless. I'm not sure how, but I'm sure you might be able to use a scanner to figure out what frequency they're broadcasting on. I know we sometimes used to get CB transmissions of trucks passing in the area. Once you have the frequency, you might be able to find a way to disrupt or broadcast over it. Pump an mp3 player into a transmitter and broadcast your own talk. Maybe the audio from one of the TV exposes on the JW pedophile scandals?
Or with the right production values, do an Orson Welles, War-of-the-Worlds style thing. Have an angel interrupt the meeting to tell them that Armageddon is beginning now. Do not leave the Kingdom Hall! Continue to break in with periodic updates on the fall of the world. With a schedule of who's on the parts that night, you could tailor it, like "Pardon the interruption, brother Smith".
Pocket all of the time sheets so people can't turn in their time at the end of the month. They will have to order more, but it could take a while.
Tape out-of-order signs to both bathrooms. Or just one that says, for instance, "Mens room out of order, use womens restroom". Or vice-versa. Put it on the door that you wouldn't see first so that some people won't know the other bathroom has a sign.
Our hall had a knob to lock the main door on the inside. Sometimes before the meeting it would accidentally get locked and people would be locked out. Might be funny to do that in combination with a sign on the outside that reads "Closed for private party".
Keep calling the KH on your cell phone during the meeting and hang up before anyone can answer. Of course, you'd have to somehow block caller ID. That's always disruptive.