After playing around with foobar2000 a bit lately, I came across m-tags. It is interesting to say the least and for my purposes I find it useful for file formats that do not have native tagging support (90's tracker modules, game sound files, etc). Foobar2000 doesn't allow one to manually enter in tag data for such files unlike Musicbee. But I digress.
For what you (and others) are seeking in Musicbee, it is probably best to use custom tags (Preferences > Tags (1) ) and arrange those as your main display tags. Of course, doing so may compromise last.fm scrobbling, last.fm loved tracks, etc because those features will most likely make use of the native tag fields <artist>, <album>, etc (correct me if I'm wrong anyone) and ratings will write popularmeter data to your tags.
P.S. I wrote this because this thread was the first hit on the google query: 'Musicbee "m-tags"'