Many thanks for you comment @SIMULACRA, it’s good to hear your experience with the bag. I am accustomed to using a quilt, having used a EE Revelation for many years. Under the conditions that I use it (typically 11,000-12,000 ft in the Colorado mountains), I’m never in a position to open the footbox of that quilt, so the closed footbox is right for me. Another standard quilt would probably work for me, but I’d like to improve the cold-weather experience just a little bit, if possible. In the unlikely situation that I will be camping in warm weather, I can still use the Revelation, but it has lost a lot of loft over the years and I also seem to be sleeping colder as I age.
BTW, I contacted the manufacturer about this idea, and the person initially responding to email was very responsive, but just said there was no way to attach the bag to a pad. When I persisted, she passed my second email along to the owner, Joe, who was very helpful. He gave me a link to the plastic snaps that they use to attach their strap to their quilt, and even a link to a cheap snap-installation tool on Amazon. He also answered my follow-up question about the best way to install the snaps.
I’ve ordered the bag and strap, and I’m going to experiment with this idea. At first, I won’t modify the bag itself, instead I’ll just attach some snaps with small mitten clips. If I’m happy with it, I will add a couple of snaps outside the zipper. Joe suggested that I might be able to attach the snaps directly to the bag material or zipper fabric. I will take a look at that, but I’m a little worried that the snap might pull through the delicate fabric or interfere with the zipper. So if that approach doesn’t look plausible, I will sew tabs of grosgrain ribbon outside the zipper (maybe through the thick zipper fabric), and put the snaps into those. That would also give me something to hold onto to undo the snaps.