In the BBC news article To see finally the face of Peggy (Peggy is a very small moonlet interacting with a ring) there is an audio interview with Carl Murray of Queen Mary University of London. After about 08:45 he discusses the masses of the rings, including this (approximately transcribed) sentence:
And that’s what you get with Cassini, in the final orbits, the Grand Finale orbits, we will get the mass of not only the rings, but the mass of the B ring, which is the most massive of the ring system.
Will this be enough of a constraint to interpret "get the mass" as an approximate mass measurement? How is this done - by sampling and characterization, or through actual gravitational interaction? If so, actually how?
The article also links to Linda Joyce Spilker's talk abstract P33B-2136: Cassini’s Grand Finale at the American Geophysical Union's 2016 fall meeting.