Well there is actually an Airborne Board at Bragg that set these kinds of rules for Mil jump ops, not just MFF. So you start with what's allowed by big army and you go from there. Then you have your local units and what their additional SOP's are. Then you have your team and what their SOP is.
In this instance, an individual team may have their own SOP concerning mag pouches- in general- not just for airborne insertion. For example, my buddy's first team had an SOP of top flap mag pouches only, so that's how they rolled- all the time. Obviously you get a lot variation on this theme. On subsequent teams they ran all sorts of stuff, including open top, kydex insert type pouches.
To answer the question, I think a KYWI would probably do just fine, based on my jump experience, and also from what my buddy has done. It is probably not allowed, or encouraged, simply because institutional experience will err on the side of caution. On a "normal" jump, you would probably have no issues. If something did come up, then there is a chance you'd lose some mags. If you went "Z" (out of control) which is very easy to do with ruck and rifle; if you bumped into a team mate; if you had a malfunction and had to cut away; if you had a hard/bad landing. You would weigh the risks versus rewards, and plan accordingly. If I thought it might be a hot DZ I might come down ready to fight. If the chance of contact is low then maybe not.
Back around the '05 timeframe I know the LF "Warhammer" was jumped with just 1" squares of Velcro holding the mags in, and they did just fine. Don't think it was ever approved by the Jump Board, but it worked.