Vanessa Williams signed regularly when she was in Sondheim on Sondheim, so I'm not sure why she's not doing it now. Condola Rashad also signed in the past.
In terms of the stage doorman, I'm guessing he's told not to make the announcement that people are not signing until after they've left the building, which in this case took over an hour. Kind of stinks for us normal people who respect the actor's decision not to sign, but there are enough crazies out there who will try to run around and stalk the back exits that I understand why they don't want to say anything earlier.
"What was the name of that cheese that I like?"
"you can't run away forever...but there's nothing wrong with getting a good head start"
"well I hope and I pray, that maybe someday, you'll walk in the room with my heart"