This is an interesting idea... It would have to be one that wouldn't trigger the cult personality, and also completely verifiably true. One mistake or half truth and the dub brain will discard the whole thing. If its too aggressive, they will get defensive.
They would probably be watching it after a discussion of sorts, so one would have to judge by how the discussion was going how the person will respond. If its a spouse, it could easily turn into a fight. A fight like that does huge damage to the process of waking up, and makes it harder to breach the subject in the future.
... I can think of one. Perhaps others can? Maybe an objective viewpoint on a matter they feel strongly about? Even losing a case for child abuse often won't click in a dub brain. It's unthinkable for their beloved org to have any culpability in something like that.