I think just knowing ex muslims exist is an eye opener for loads of muslims.
First time I found out about ex muslims I was shocked, it just never really occured to me that there would be ones

although that was kind of stupid because my eldest sister left islam decades ago and converted to Roman Catholicism.....but it was because she chose another religion that she never counted in my head.
I mean the idea of leaving Islam to be another religion never tempted me as they are all bullshit, but becoming an ex muslim atheist, learning they existed, that was something amazing to me. Like a key that opened a part of me that conditioning and abuse had locked away.
I don't believe that the campaign should be so challenging though, a lot of posters I have seen, or booklets I always felt focused too much on instantly trying to show the winning non religious side to an argument, how about just letting people know we exist? that we are here? let them learn something they may not have even thought was possible.
I even met a girl in a club once who was a muslim, and when I told her I was an ex muslim she was so fucking shocked too, just like I had been. The idea that we even existed was so much for her that she wanted to debate straight away, she wanted to know more. (funny story, she was hitting on me, promising to do some tempting naughty things, that she totally didn't want to do anymore when she realised I was an ex muslim

)