I apologize if this sounds a little patronizing; it's not meant to.
However, while you're techincally correct about the space blanket reflecting heat back towards you, the vast majority of what a space blanket does to keep you warm is prevent evaporation (i.e.: it's a vapor barrier).
Something like 95% of your radiated heat (what the mylar in the space blanket actually reflects) is absorbed by your clothing. Which means that only ~5% of your radiated heat actually reaches the space blanket to be reflected. And, the majority of heat loss across normal temperature gradients (from your body temperature to the surrounding environment, say a 120* F difference at most) is going to be through convection and conduction, not radiation.
What a space blanket does, though, is prevent the evaporation of vapor from your skin. It's the same as wearing a trash bag (or non-breathable rain wear); it makes you feel hotter since the state change of the water to vapor isn't carrying away any heat from your skin.
So, mixing in pieces of space blanket into a down quilt won't do much, if anything at all. More down would be a better use of weight and bulk at that point.
However, if you were to make the inside of the quilt one large space blanket (or other non-permeable material like sil or cuben), you would get perceived warmth out of it. You'd also wake up wet, which is the downside to vapor barriers. They take a lot of skill to use properly (and I don't pretend to have that skill).
Hope it helps!