Highlight tool is just a feature which dependes on the field "Lyrics?"
For your first question, maybe if you set a common tag for all of them, maybe composer or comment could work. But you would be assuming a high risk of breaking tags and/or lyrics as commented above. Doing it or not, backup always your full library.
The field "Lyrics?" i understand is well reasoned stored in the Musicbee internal database, as the lyrics we are talking about are stored externally, not embedded in the file. The database also stores the number of playcounts, date added, etc