Well.. unfortunately it turns out that it won't really matter how I record the choir, they're more interested in shooting the ad itself. They do want a recording of the choir, but the main focus will be on what's being spoken in the ad... I got the song that needs to be recorded, it's 40 seconds long... so you can imagine it's a big let-down for me.
The client's main focus is on getting playback in sync with timecode to sync in post-production, so you can imagine.
I will post pictures of the hall, of course, and the recordings as well. Not sure it's worth investing extra money in renting microphones for this though.
Thank you for all the feedback!