The shape and taper of the flap may negate the need for a snap at all. I had a concern with my full-length closure, but you may not have as much to worry about. Smaller surface, less chance for snag. The flipside being- smaller surface, less chance of retention during a snag. It's a balance like anything else.
About the only buckle I would put on a TQ pouch would be a snapdragon or one of those National Molding deals with the pulltab, but I don't know if I've ever seen them smaller than 1in, and thats a lot of bulk and complexity for such a small/light pouch.
I have never received a report of the D27 having an issue with the TQ flap inadvertenly opening (not saying it hasn't happened, but no one who knows I was involved ever mentioned it to me), and none of those that ever saw the light of day, three or four versions, had a snap. Most were used high on the chest as intended, though. I went to a snap when the stand-alone matured enough to be built. Everybody wanted to put it on a belt and I just didn't trust the velcro-only.