Last November I saw Naken Girl on the Appian Way, then ran over to the Golden and was still able to buy one of the "It Sucks to Be Me" buttons. So it is quite possible to get stuff from shows you didn't see, but you can't hover at another theatre. Once the audience from the show leaves, they pack it up and go.
Good luck! I'm seeing Wicked next month and the cookbook sounds like fun.
"If you start from a place of joy and charm, you can get away with a lot in the second act. It may be that that's my life. I'm not sure."
~Roger Bart