I think it's linked to the KQ9 source. Three players, helping each other out with the puzzles. It probably wouldn't work out that well, but it could work. I don't know much about how the Royal(anyone know their last name?) family will work in the game, but I doubt they'd all be playable at the same time. If they were, it would probably be simple to implement the network code(not writing it).
P.S. I thought of another CLOSE one, the Quests in Balamb Garden. They have mini adventure games(quests), and they're not bad. Sadly, the multiplayer Quests are more like RPGs.