The late lamented "Frets Magazine" did an exhaustive analysis of the brass nut phenomenon at the time. I don't know of an analysis of bridge pins - its easy enough to try it yourself - although it does make some sense. Yes, the vibrating mass is the whole top but the heaviest part of this, which receives the initial momentum, is the bridge. If you add mass to this, it will, at least alter the characteristics of the sound. The received wisdom, which came from an experienced UK luthier, is that it makes it brighter.