Neil Patrick Harris has signed on for a special guest role in a traditional British panto staging of “Snow White” in Hollywood. The “How I Met Your Mother” star — now holidaying in the Med with Elton John, David Furnish and their respective babies — will play the Magic Mirror. Marina Sirtis of “Star Trek: The Next Generation” will star as the Wicked Queen. The musical-comedy production, at El Portal Theater from Nov. 30 to Dec. 18, will be helmed by Bonnie Lythgoe and choreographed by “Dancing With the Stars” pro Mark Ballas.