Aha – have just found a youtube video showing what you describe. So I guess this inner valve would also be ‘sticky and cling to itself’ as you mentioned before on your own gear. I’m dead keen on finding a replacement for zippers if possible, and was just thinking about making up a test rig of a bladder representing the hull with a ‘tube’ thing in the seam. It would look quite like the neoair inner tube, but 75 or so cm long and high enough to get say 5 rolls at 30mm each. And then velcroed down to the hull with a flap velcroed over the whole thing to tie both sides of the hull together. The cargo zips I have now are 72cm, but the one on my MRS raft always niggles at me – it’s too easily damaged and means that the raft is no longer field repairable.
I’ll postpone making this until you’ve tested yours, if that’s not too cheeky:) I now see that this inner valve seals by virtue of using pressure (rather than fighting it), and having one hanging down as you say could well be key.