Can't see why that wouldn't work. Sounds kinda like a cold frame -- a move-any-where cold frame!

I do my WS'ing in my unheated plastic-covered greenhouse. So far I've been happy with the results. Most of the stuff that sprouts earliest is the hardiest stuff anyways that survives and even thrives when the temps go down. Last year I had lupines, dianthus and venidium keep sprouting during a spring "cold snap". I did have some zinnias up already too and only 1/2 the container died and the other zinnias which were a little more protected pulled through just fine. Pretty much everything else started sprouting later anyways.

Crazy, watch when the snow is melting that you drain your tray regularly, or even take the pots out. Otherwise they'll get too wet. You'll also need a way to prop the window up as the weather gets warmer. If you were able to make an actual cold frame and use the window as a lid, it would work better, i think.

Shauna has mentioned using a blanket bag for some of her containers, and that might be better than laying a window on top of them. What i'm thinking about is that when sprouting starts, things sprout at different rates, and some of the sprouts might grow too close to the glass. With a blanket bag, the plastic will be high enough above the pots (you can even devise a way of propping it up) that the sprouts can grow on until you can remove the plastic.

Most of my winter sowing is done in milk jugs, plastic juice bottles and clamshell containers (eat LOTS of strawberries!). They all come with their own built-in lids, which is easiest for me.

Good luck with your winter sowing and let us know the results! :)