I haven't had direct experience with that either, but I do know what it is like when the priest is close to you and he is singing loudly and at a completely different tempo from what you are playing and he hasn't clued in that he is way off. I just tuned him right out, and stuck to my tempo with the congregation following me.
The metronome idea is a good one. We sometimes get all kinds of distractions as organists. Here are some examples I've encountered:
- A lady walks up the chancel steps as I am playing the postlude, and she misses a step and falls down only a few feet away from me. I turned to see if she was okay, which she thankfully was and there were people helping her up. I managed to still play my postlude without having to stop.
- In a church I was organist, I came in to practice and soon came the custodian with the loud vacuum cleaner.
- I met with a couple in the church sancturay to discuss music for their wedding. I didn't know at the time that another congregation were using the large hall below us. I start playing some suggestions for their special day, and then their praise band starts and they are so loud, we could barely hear ourselves talk. Talk about serious risk for hearing loss for those downstairs. Then they stopped and I thought it would be quiet for a bit. But then came the sermon, and he was so loud I could make out everything he was saying.
- Someone decides to strike up a conversation with me while I am playing either the prelude or the postlude.
Oh well! 
Actually, maybe I should make a new thread on this..."what kinds of goofy, weird, or surprising things have happened to you while playing?" See y'all in the General section!