The only insulation I would suggest putting up there would have to be very light weight. The headliner is set up to only hold itself up, and not much of anything else (it's only cheap cardboard). The only points it is secured at are the corners and where the light in the center is located. Anything heavy and it will give.
Oldspartan, don't even bother with any redneck remedies. On my old Camaro I had a sagging headliner and being a kid with little money I tried everything. I tried glues of all types, and even staples, but nothing would work. Finally I broke down and found a local shop that redid it for $100. After that, I had no more problems. Call around to local shops and they can probably redo your old one cheaper than getting a new one, and they will probably match it up just fine.