On the other hand, as Doone pointed out to me privately and as Bigsky has posted, a celeb could certainly use their vanity kewyword to take calls that updated the caller with a recorded preview of a new track on an upcoming release, a concert date or some other info of commercial interest.
I don't imagine that any celebrity would be turned down if they expressed interest in a Go800 vanity name other than their Twitter username, particularly if the user name is too cumberome to text.