haha h ah ah aha ha h ah aha ohh homey!! you slay me !
fuck yeah it would work, pop tarts are wrapped in Mylar which is the doper's preferred reflective material! I like the way you think, but tacking up all those little bits would suck balls, you would have to use spray mount or something to stick em up, and you'd have areas of poor reflectivity where they join and overlap. plus after harvesting a few hundred pop-tart pelts you would not fit through the door of your growroom! seriusly bro, plain bright white paint would cost you less ($3 a pint, one time) than hundreds of pop-tarts and your gastric lap band surgery!
shit son, pop tarts, thats devious and clever as fuck. if i ever get locked up you're the fucker i want as my cellmate so we can shawshank our way to freedom using 2 paperclips a rubber band and half a toothbrush! on a similar theme you might consider dumpster diving for chips bags, the big ones are often lined with Mylar, and after a quick wash could be used the same way but with fewer gaps, overlaps and ingested calories.
Pop-Tarts. pure genius.
really though, white paint is almost as effective at light reflecting, and is almost free compared to the effort and expense of buying and hanging mylar, or fishing chips bags out of dumpster, or clubbing all those baby pop-tarts for their skins.