OP following up here...
Pic below is the received item with the goods inside. As you can see, all of my specs are met -- the water bottle will go in one side pouch and (if needed) a tripod or raincoat will go in the other pouch. I may just double down on the water, though.
That item again: http://www.lowepro.com/flipside-sport#prettyPhoto
(I got the tiny 10L)
I could swap my 70-200 F/2.8 IS II pretty comfortably if I switched dividers, but the hat may have to go. But the pack is small and centered on my back, and it's very size discourages "over-gearing" for a hike. I kind of love that.
Thanks for everyone's advice.