I use an KBC VR-X snowcross helmet in winter, which has a breath guard mask installed in it. It also has a strip of aluminum that can be pinched around the nose for a good seal, and it allows the breath to escape below along the neck line, instead of letting it fog up the shield. And it works great! Combined with the dual-faceshield setup on the KBC, it completely eliminates fogging in cold weather.
This helmet basically includes all the cold weather riding features from the get-go.
Thanks for pointing out the separate mask solution. If I didn't already have my VR-X helmet, I would certainly put together a combination of the FogCity visor insert and this mask that you found.
It looks like that company, Respro, also makes a very interesting rain suit that fits tightly over the racing leathers. I'll probably give that a try next season.