I've got a CC bag- a 32oz nalgene fits, but it takes up most of the bag. You'd loose gear trying that.

Are you planning on carrying this by the handle, around the waist or over the shoulder? Another possibility is using the molle straps on the back to anchor a pouch for one of the small bladders, say a 50 ounce bladder in an Emdom SUV pouch or something similiar, which will ride fairly flat, particularly if turned sideways and annex clipped into place.

Or you could just add one of the cheapie water bottle carriers from coglan's- they adapt to fit anything, and they cost more to ship than to buy.