Wtg RMC33!! Will you really expose your 400 to heavy snow, or do you have a bag or blanket for it?
I usually just toss my ski jacket over the whole thing as I hate shooting encumbered. Snow generaly just piles up due to the white coating on the lens not heating up. I have had days where my 7D would be warm to the touch (32 F sunny) but the lens was same as ambient and as such no melt factor. The Nikon shooter I know uses a lens cover because even on freezing days the sun can melt wind blown snow on the big black lenses.
Excellent point, I hadn't even thought of the snow melt factor, I was just thinking of a heavy amount trying to cram its way into the cracks, especially at the camera mount.