My thought is that if you can't find something, assemble something of your own to test the concept.
Sorry for the cell pic...
While not for an AR, I used Eagle m9 extended mag pouches and an old 5.11 chest panel.
My only complaint about my 9mm setup is the chest panel is a little floppy but it does breathe well.
=)
That setup from usgruntgear looks pretty sweet!