I remember a story about a magician who did a gig at a church. Because his community didn't allow playing cards in the church, used blank cards instead and just used the audience's signatures as indication.
In my memory, this magician was Harlan Tarbell, but I could be wrong there.

So if this sounds familiar, please let me know where I can find this story retold originally.

Unfortunately I am not able to reference the story but I might do a routine like that which represents a reference to names appearing in the Lamb's Book of Life.