Here's my current favorite candidate for an adjustable buckle that works on 1/2" grossgrain. It is similar to the buckle set-ups commonly used elsewhere (e.g. belt buckles).
No field testing yet. There is the possibility that the thin 1/2" grossgrain will distort to the point of it not working. I currently plan to try it on a tension point on a pack that will not require frequent adjustments.
The two 1/2" id rings are some cheap plastic ones I got at a 2nd hand store. Two of them weigh zero grams on my scale. I'd prefer something made of nylon and will be looking for such if these work.
I tried D rings but they rotate around the hanging web and things get tangled and jammed up.