These might be the ticket. They look like they place the bottle pretty centrally over the bolt holes, but they also have some room that it looks like you could drill yourself some new holes a little higher up to squeeze another 1/2" or so of room out of them. Or if you're really desperate, drill a hole somewhere in the middle of the mounting plate, bolt that to the lower water bottle boss hole, and then just hose clamp the lower portion of the cage to your seat tube...
http://www.treefortbikes.com/product/333222372576/553/Dimension-Sidemount-Cage.htmlMy other advice would be to just make the leap to a full frame bag, which then could hold a larger bladder, or even two separate bladders, not to mention tools, tubes, etc...