Most of our distance races exclude that rule for the event....or at least they should. Although, I don't think I've ever actually seen the exclusion in the RSIs, most of the committee's I have asked about say that it is allowed for our distance racing.

I have a lifejacket that was a limited run built for this purpose. However, the pocket in the back is part of the outer shell of the life jacket such that when you fill it with a water bottle, the lifejacket gets about 1 size smaller. It is nice to have it all together as one unit but I will be using that lifejacket and sewing on an 80oz camelback to the back of it this year just because it's more comfortable that way.