I know a local pipe insulation guy who does a lot of oilfield work; He makes insulating blankets for pipe and valves, etc. He can make a ceramic wool blanket any size or shape, and sandwiches it with a silver-colored silicon reflective cloth, then sews it all together with a big, manly sewing machine. (Hoo-rah!) Make a pattern and find an industrial insulating contractor. You could attach it with screws and AN970 wide area washers. (show where you want grommets or holes on the pattern. For valves and exhaust, they typically install hooks like the ones on your boots so you can lace it on with stainless or brass wire). I've used this stuff on marine exhausts directly on the pipe with no problems - good for ungodly temperatures. (I've got a 12" square that I use for plumbing hot work). The ceramic blanket comes in various thicknesses - I think down to 1/8", but my requrirements have always been 1" or 1 - 1/2". It probably absorbs sound as well, but I wouldn't vouch for that.
As far as whether or not it will be LEGAL - You'll have to figure that out for yourself. I 'spect not, but I KNOW the stuff is safe. It WILL burn if you get it hot enough, but it doesn't give off noxious fumes or any crap like that. If it got hot enough to do that, it would likely be too late! As I said, I've used it directly on exhaust pipe with EGTs to 1400 degrees . . .
Cheers