FreeZoners don't always apply "Standard Scientology Tech". You might find one that would make such an agreement. If you did, however, I wouldn't expect to make progress. Having such an agreement, about unwillingness to discuss some things, would undermine any therapeutic relationship.
I'm not a scientologist, but I would make no such arrangement. When I give my orientation/intake, I make it clear that some topics we discuss will be difficult and trying. The topics covered will be chosen by the "subject"/client. However, if they get into something that associates with a topic they are uncomfortable with, I won't FORCE them to talk about it, but I make it clear that unwillingness to communicate about a subject is an indicator of charge, and that I wouldn't expect them to be able to make progress without handling any charge that comes up.