I found a nice solution with the Inov-8 Race Pro 4 lumbar pack. It sits low enough so shifting weight is not an issue, and even the speculative reviews boasted a massive capacity. I'm planning to use it with the included reservoir so I can mount a frame bag where my bottle holders go. It's supposed to ship to me tomorrow. I can update with a review then.

Here's a link: http://www.zappos.com/inov-8-race-pr...e?zfcTest=fw:1