Actually I do believe he is more of a fan of Panerai than Brad Pitt is of Guess Sunglasses. Brad Pitt is known to be a bit of an advertising junkie - Tag Heuer, Rolex, Honda, Toyota, Sanyo to name just a few. I'm sure Stallone doesn't only do it out of the goodness of his heart, but he has been a little more discreet and he seems to have almost exclusively worn PAMs for the last decade. Perhaps without the attention of these "white-trash" action stars, there wouldn't be nearly the amount or quality of PAM reps available, so, "every cloud..............."
Despite this, I don't think he needs another signature PAM, I'm sure he could find something suitable already produced.