Putting the tags in the library is the safest and most well known procedure. It won't even take all that long, though MC necessarily will have to write to every file in your library that has any tagged fields that are no already saved inside the file.
The other way to do this that I know of is using the "Export Playlist" and "Import Playlist" functions. MC has a playlist type called "MPL", which is a form of XML. If you select this type, you can do it from a view that shows only the fields you want to show like Album, Artist, Name, Rating, etc. I would include filename as well. Once this is exported, you can re-import it into a fresh library and it should populate those fields only.
MPL export/import is a little bit weird though. I've occasionally had to do the import TWICE in order to get the fields to populate. I have not experimented with it in several years now, so my memory isn't all that sharp about it. You might do some searches and read old threads about it.
It's honestly much easier to just save the field or fields to your media files. But you can try this. Hell, you can even build a test library to play with and leave your main library intact. That way it's totally non-destructive.
Good luck.
Brian.