Lest there be any misunderstanding, she has confirmed that it will be a "live vocal", i.e. not mimed, but it won't be a "live performance", it'll be a pre-recorded insert.
Which is fair enough, really - you can't expect her to get out of her "sensible judge" clothes and hair and into Pop Tart Gear And Barnet, and back again, in the tight time-schedule they have on that show.