Use a crop cam.
When I am forced to sit at the very back of the church, the 40D + 70-200 is the primary setup. This is one of the many reasons I will not shoot strictly full frame. I want options. And a crop body that gives you many options throughout the day is much better than slinging a 200L or 300L IMO.
Here is one of those weddings. EXIF tells me I was about 70ft away from the couple. But with a crop cam, it's no problem!
And with the pixel density of modern crop cams, you can probably crop the image to 2X (or more) and still come out with a very high resolution file. 400mm or 500mm F/2.8. Boom. There you go. This setup is also super useful in reception sniping.