The tune the piper plays is the same tune used in The Real Ghostbusters episode "Ragnarok and Roll" to bring about the end of the world.
Among the ghosts that attack the NY Tour bus and the gala at the Metropolitan Museum of Art are the five specters seen surrounding the Ghost Train in the opening credits. The most prominent one, who gets itself photographed before the opening credits, is known as the 'Lizard'.
Eduardo notes that the Piper 'talks like a bad Robin Hood movie'. The Piper's voice actor, Roger Rees played the Sheriff of Rottingham in Robin Hood: Men in Tights (1993).
The curator of the Metropolitan Museum of Art first seen in The Unseen (1997) appears again, and is revealed to be named William.
When the Piper first uses his music on the ghosts, Bruno the Pawn Broker from Casting the Runes (1997) can be spotted hiding behind a taxi cab.