The one I have from Logitech uses an optical sensor for the movement itself. Gunk still builds up in the small balls that ensure the trackball itself rolls smoothly, so those have to be cleaned regularly anyway.
Yeah, OP author mentioned magnetic suspension of the ball so I was kind of assuming that. A challenge with the magnetic suspension would be the "floaty" nature of it. You would have to filter out the noise of the ball itself getting moved around a bit as you rotate it.