Filtering on missing album art in wma files

Hi, I'm trying to find songs in my 45,000+ collection that do not have embedded album cover art, so I can add it to them. They're all wma files. I saw another thread [which I can't find now] that suggested creating a new column for covers using a regular expression w/ %covers% or similar in it but when I did that the column was not populated by the expected 'yes' or 'no' for album art.. wonder if that was only for mp3s vs wma. Thanks -