I currently use handlebar bottle holders on my bikes without braise-ons and also on the ones that only support one bottle. It doesn't look great, but it's functional. A camelback would be great for going farther but I don't think I could stand wearing the gizmo while riding.

Can you extend the hose so that it could be mounted to a rear rack?

...hmmm a front rack might be the answer for this as well. uh oh, now you guys are going to cost me money.