Do what I've done: Just walk in, find the "back room with all the elders" and ask them which frequency and where a good place is for you to record.
If you cannot pickup a radio signal, just find a folding chair in the corridor (always plenty) and sit next to one of the small speakers and place your recorder right next to the speaker.
You'll get everything, including the songs and prayers.
Note: When you ask about the recording information, they might start interrogating you about what congregation you're from and so on. Just tell them you're from the public. If they tell you that the convention is not meant for the public (they tried to tell me this), then show them a printout of the WTS website inviting the public (which I had on had).
Be prepared to counter their BS, all the while act like you're just a curious person who wants to learn more. Keep it light and just ask like you are a curious person. No need for any cloak-n-dagger nonsense. You could also tell them you are doing research and will be writing up an article to be published... which is true because you'll end up posting about it on this site.