Gotcha. Yeah, you'll need consistent and uniform temps for a good mash. Our tun drops like 1 degree over an hour, and since there's no heat supplied, it's relatively uniform. Too hot and the enzymes fizzle out, too cold and the shit stops.
Now that I think about it, I'd also be concerned that you might get some off flavors if you use it for hops. I forget the chemical, but one of the bad ones boils away, which is why you never want to cover the pot. With that bag over the top, you might be catching some.
I dunno, you've done the research and I haven't. Just some shit I've learned the hard way over the years.