I just carried a Tokina 80-200 f2.8 through DL for 5 days along with 3 more lenses (Canon 10-18, Sigma 30 1.4 and Tamron 17-50 2.8), my 40D with attached grip and a usually filled 2.5l hydration pack. The Tokina is comparable in weight to the Canon (it's a complete metal lens - even the hood is metal...), the Sigma is 15oz, the Tamron 17oz and the Canon 8.5 oz - just the Sigma and the Tamron would be the weight of the extra Canon zoom you are looking at.
I didn't mind the weight of that setup at all - in fact I hardly ever noticed I had such a load on my back until I took the backpack off my back. But then I have to say I have a backpack with a very good suspension and strap system (Lowepro Photo Sport). I even added a 3.5 lbs tripod and 3 cans of Monster to the load for the 24hr event. I think it really comes down to how you are going to carry it? I wouldn't want to carry that weight in a messenger, slingbag of regular backpack the whole day for sure but if you have a good suspensions system, why not?
I even saw someone will a completely filled up Lowepro Rover 45l at DL. I snuck a peak at the load when he changed lenses right next to me. Now that I wouldn't want to carry the whole day, even with a good suspension system....