The internal pockets are ACTUALLY, just Velcro openings at the top of the pouch created by the fold of the fabric... essentially I'm saying it's open empty space...
What I can say is that it was intended for storing maps, paperwork, etc. Flat stuff... I've tried even running my death rag in the pouch on the 1 piece and I find the extra bulk tends to push into the body and not away from the body... Just a thought on that. Also if you wanted something made or built, it's suggest talking to the guy from Shaddox Tactical (here
http://www.shaddoxtactical.com/ ) he's a local guy, and I've seen some of his work. It looks pretty good. I discussed doing some mods on equipment and he seems to have some good ideas and wants you to like the item more than just wanting to sell it to you (if that makes sense)
As for the 1 piece VS the 2 piece both definitely have their selling points. The 1 piece is, like ogre said 1 solid piece of MOLLE goodness... This however makes it more difficult to Don and Doff, especially as you add accessories like the Bib and the X harness... However I did a simple Mod on my X-harness which makes getting my MAV on and off a snap... (Now I just have to learn to remember to take off my ear bud for my radio BEFORE trying to remove my rig) The 2 piece makes on and off easier and is a quick way of getting your gear out of your way if you want to go prone. You just open it up and not have to worry about what's in the way on your chest. (This is actually why TT designed the 2 piece.) However the 2 piece cannot be used with a TT MAV Bib. The buckles on the 2 piece are set closer together creating what I call the Floppy Bib. As seen here...
As for more space I can understand both arguments. They both have 3 rows. The 1 piece has 18 columns, and the 2 piece has 2, 9 column panels. This makes the 1 piece more versatile. Mine for instance has 4 shingle's centered with 3 Dbl pistol mag's covering them. Making a nearly impossible setup for the 2 piece. However TT does sell this
http://www.tacticaltailor.com/mavcenteradapter.aspx Handy hook and lop, center adapter, which actually covers the buckle section of the MAV with 2 columns of Mole, making it larger than the 1 piece. (however my setup with the 4 and 3 STILL would not work. And that handy item will run you $22 for 6 MOLLE loops

)
I do recommend the X-harness. It makes carrying any amount of weight a lot nicer on the shoulders at the end of the day. The bib is nice too, it adds and easier accessible MAP pocket, on your chest and could nicely hold and Admin pouch or what not. However it has no retention strap on it, so it tends to flop a bit when any real weight is added to it.