I actually don't believe this to be the case. Based on some research and comparison to signed CDs for Bette's "It's The Girls" sold on Amazon, per a special Amazon exclusive, they seem to be authentic. Also, I spoke to the Asst. Stage Manager, and she assured me that the cast did gather to sign, and that all signatures are authentic. Furthermore, the reason for the recent delay in sending out more signed Playbills was because they ran out of the initial batch right before the Tonys, and because of the post-Tony buzz, the whole production put signing Playbill for BC/EFA on the back-burner.
In other words, I believe Bette's signature to not be secretarial. :)