The MAV's shape is held better if you have a triple mag pouch/3 singles on the front. I have a single and a double open topped pouch with my Admin panel on the front keeping them rigid, and the panel's pouch is also good for speed loaders.
I also wear a camelback with a 3L bladder which lasts me all day.
I think you'd be much happier if you used a yoke on a belt rig as it distributes the weight of mags, water bottles etc between your hips and your shoulders. The battle belts have a tendency to slip down without them.
I don't think there are any battle belts/belt rigs designed to be attached to trousers, as users would generally want something that can be taken off quickly without fiddling around with their trousers.