Over the seasons, characters have left the show, only to reappear. Rachel Menken appeared as a recurring character in season one, and was not seen again until almost the end of season two.
Paul Kinsey, who had not been asked to join Sterling Cooper Draper Pryce, was absent during all of season four, but appeared in season five. Based on this pattern, it seems possible that Sal may make an appearance in a future season, if only for one episode.
Under what circumstances do you think Sal would reappear? Would he still be with Kitty? What would he be doing? Would he still be in advertising?