The TT pouch has a grommet- I just got two to carry real water bottles rather than the minis I've had in the ditch kit for years. Can't comment on the canteens, but they hold a Nalgene or Goyoet (sp) and GSI or Olicamp cup perfectly.

Have you checked out the Maxpedition M-4? It might worth looking into.

http://www.maxpedition.com/store/pc/viewPrd.asp?idcategory=6&idproduct=157

And if you really need it custom, Tactical Tailor and High Speed Gear Inc are both good people to talk to. I know of several one of a kind items they've done for people. I almost had them make me an X-harness for my MAV-2 that would have had hydration carrier built in, but it turned out that it wouldn't work so well with my habit of wearing my MAV backwards as a lumbar pack when I don't have a bigger bag.