... I may get away with small pieces of AF ATG... any other ideas out there in the framing ether?
Since you've patiently waited almost a day with no other suggestions, here's one.
Use hinges of 3M #887 tape. Like Mylar, it is made of clear polyester film, and it has a very good acrylic adhesive. It adheres to clear polyester film better than anything else I've seen so far, and it is so thin that it probably is the least-visible choice.
You could cut a window in a sheet of Melinex 516,matching the size/shape of the artwork plus 1/32" on all sides, and hinge the art in the window with a couple of pieces of #887 tape.
Suspend that Melinex "window sheet" in a larger window mat of 4-ply mat board. If it is to be a recto/verso framing design, then make a recto/verso window mat of back-to-back 4-ply boards.
If the edges have to show, how could you use ATG tape? Using any double-sided tape, you would have to overlap the two surfaces, right? Even if it would work for this job, ATG would be among the last of my choices, because it is thick and has no carrier. It would be quite visible and it could migrate, especially on the slick surface of Mylar.