Import of (what's left of) http://bugs.musicbrainz.org/ticket/702

"ft.", "f.", "ft", etc already turn into "feat."

"f/" should do the same, as shown in http://www.mixtapesusa.com/lilwacryomof1.html or http://www.mixtapesusa.com/neweacohoofg3.html
I don't think it is commonly used in any other context, so it shouldn't be a big problem.