Twinkle- I wondered about that cause when they released DD2 from the NICU they told us no to any extra things on the carseat (I didn't have any because they make me nervious. Just a random carseat question my hubby and I were debating: Jackets/sweaters on a 3 year old in a carseat would be a no as well right? DH says it would be ok, I say no because of the compression causing the straps to fall off in an accident.
You can make a blanket to go over top and velcro to the carseat handle really easily to keep the wind/snow off the baby if she/he is in the seat. Im making one to keep the sun out of the baby's eyes. Kind of like this: Car Seat Blanket
only Im making a few minor changes (the ties totally wouldn't work with my kids, they would be able to tear them off starting at like 2 months). I figured it would be safe since it isn't wrapped around the baby. I haven't tried the one I designed yet but when I do Ill post a picture, Im trying to wait until I find out the gender but I probably won't be able to wait that long.