Sorry I didn't see your post for such a long time. I think this is what I did.
I didn't find a regex expression to do the parsing of the comment field within MC when they were all out of order, so I used MP3tag. It is one extra step, but really not too painful.
In MP3Tag you can set up an "action" to copy the data from the unreadable field to a custom field which we can then get MC to read. The action is a "format value". Select destination field, in my case I named them .LASTFM DECADE, and the "format string" is the field from where you are getting the data, e.g. %Comment Songs-DB_Custom1%.
You set up one of these actions for each of the .LastFM Decade, .LastFM Category, .LastFM Country and .LastFM Occasion. Select the MP3 files, choose the "actions" to operate on the files, click OK.
You make the same fields within MC, setting them to string or list type as appropriate. I am never sure whether MC sees external changes to the files (I presume so), so you may have to update library from tags to get it to flush the changes. Add to your views and you are done.
I think this is what I did, and what I have to do to any newly processed files. I don't think you need to create the .LastFm Country, etc fields within MP3tag, but not sure now.
Let me know if unclear or if I missed something.
Craig