If you are planning for an emergency and not for daily use, I would say two of the heavy duty gargbage bags (4 mil/>) are a minimum (and probably the cheapest solution, especially if you can beg a couple from a road clean-up crew); they have worked for me in a white-out situation...along with animal droppings to act as an insulator against the rock. That is taking into consideration that the clothing on your back is your primary shelter system. Gore-tex bivies are a good minimal type package, too.

Mylar space blankets tear easilly, which would negate their use as an insulator (air-trapper) or windbreak.

The Blizzard Survival Sleeping Bag's Reflexcell technology appears to be three layers of mylar with a reinforced baffle system: http://www.blizzardsurvival.com/page.php?xPage=reflexcell-technology.html which should be good at trapping air when the walls are allowed to seperate. If you sit/lay on the bag, it appears the walls would be pressed together and the system would lose the insulative quality at that area, making it simply a 3-layer space blanket.

If someone like thermarest made a bag, that would be better with the positive pressure, but could still compress with body weight enough to negate insulation in that area. An ensolite bag would be even better for uniform insulative qualities, but it's bulk would probably keep it from being carried.