I haven't thought about that, actually. It does sound pretty fun. xD To be honest, though, I wouldn't let my Master Ball go to a waste like that just to surprise others. Master Balls are very rare—it's nearly impossible to obtain more than one in each game… well, unless you use cheat codes, of course.
I haven't used my Master Ball yet. I'm saving it for some sort of a special occasion. I did use it on Zygrade once, but then I started regretting it, which caused me to reset my game (which was last saved before capturing the Pokémon), and catch Zygrade with some other type of Poké Ball. There are said to be three hidden legendary Pokémon in X and Y—Diancie, Volcanion, and Hoopa. If they ever announce them officially and give us a way of obtaining them, then I may use my Master Ball on Volcanion. If I end up not catching it with the Master Ball, then I might use it on Diancie instead. (: