Our team feels your pain. I really don't see why rejoining would be restricted; after a game is finished (be it PvP or PvE) Bungie.net will record the stats for all players who were present, which means a list of players persists throughout the session until the very end. All that would need to be done is compare the username (be it a hash, PK, or whatever object/storage method is used) and compare it to that of the players who were present at the beginning of the session. If the players match, allow a reconnect. If not, then present them with the not joinable screen.