I hope this question gets a lot of answers because it is a complex situation.

My two cents are:

1) The obvious. Have a very fast lens, and I mean a very fast lens.

2) Use the bigger ISO your camera can give you. Probably one ISO less than the maximum. It is somehow easier to remove noise than a moved photo.

3) Shoot in raw. You probably can underexpose 1 step if you need that little extra shutter speed.

4) Get a monopod. I have not used them but there are some interesting monopod with a tiny 3 foot stand at the bottom. It is more flexible than a tripod.

The not so obvious:

5) Get an assistant (or 2) and a radio triggered flashes, with a portable diffuser. Not the tiny plastic ones. There are a couple of foldable models... very interesting. It is easier that one assistant moves from here to there without a tripod... he is the walking tripod.

I'm not sure if you need to balance the light from the flash or not. That will depend on your style or look you want to achieve. Experiment a bit. Try to use the ambient light as much as possible.

I would not use the flashes at maximum power but 1/4 or less. Depends on the situation.

6) I would try to shoot at 1/15 or faster. Probably 1/30

7) Try to study the location in advanced. Try to measure the ambient light to know what to expect.

8) Talk to the responsibles of the ceremony, tell them you are the official photographer and ask where you can stand. Be gentle.

9) Probably a dumb idea but it is an idea... Have a backup "point and shoot camera" — some of them are very good in low light situations. I would use it for some general views with a wide angle zoom.

10) As SailorCire wrote on the comment, get a better camera. Better low light performance is one of the best reasons to upgrade a camera body. You can probably rent it if you do not want to buy it right now.