Well I'm not too familiar with using WMP but I did some testing and it doesn't seem to be messing up my album art. There's an option some where in WMP that says to update files with info from the internet and that is unchecked and it doesn't appear to be changing anything in the files them selves as the modify dates on the files never change. It appears WMP will create folder.jpg and AlbumArtSmall.jpg though if they don't already exist. If they do exist already it appears that it just uses the existing ones. This is WMP 12, so maybe earlier versions handled things different.
To my original question, I spent a few hours on google trying to find out where WMP stores it's "Volume Leveling" data. I know it is in the files tag and found someone that said it uses tags called "AverageLevel". So I tried adding a custom field in MC to read that value but MC just shows that tag with no value in it. I'm not sure maybe I'm setting up the custom field in MC wrong.
EDIT:
I just went back and looked, the post about "AverageLevel" being the tag WMP uses was from the year 1996. So that was obviously an older version of WMP. They may or may not have change how they are storing that info.