This is such a good idea! Non-accessibility of theatre and cinema screenings has been an issue for several of the famlies that I work with, I know that several will be keen to give it a go.
As an aside, I have Crohn's disease. I love going to concerts and to the theatre but couldn't go for ages, in case I needed to nip out to the loo (they won't let you back in!). 'Relaxed' performances would have been a godsend to me at that time.