Something else to consider: Person X believes that gay sex, or maybe abortions, are immoral. Is it okay for him to show people pictures of those activities in order to shock them? As in the vegetarian example, the imagery adds “information”.
But I would certainly classify trying to get someone to oppose gay sex by forcing them to confront how disgusting it looks (to non-gay people) as “immoral mind-hijacking”. It doesn’t add meaningful information. Technically, you could claim that someone wasn’t aware of how disgusting it is and now that they know, that’s new information, but that’s not the type of information that’s normally relevant to rational arguments.
You could also say “well, some disgust isn’t rational, but the disgusting aspect of animal slaughter is a different kind of disgust which is rational”, but you’d be hard pressed to come up with an argument for why the two kinds of disgust are different that doesn’t amount to a direct argument for vegetarianism that could be expressed without involving disgust at all.
I think that if I were in Nazi Germany, it would be acceptable to forcibly show people videos of concentration camps to save Jews. However, if I were in Nazi Germany, it would also be acceptable to cheat, to destroy property, or even to shoot people in order to save Jews.
To a rational vegetarian who thinks that eating animals is as bad as killing Jews, it would be moral to do anything in order to prevent the eating of animals. However, such a variety of vegetarian is a menace to society from the point of view of other people who don’t share in his ideology, and would (according to those other people) need to be stopped—and rationality doesn’t really matter at this point; whether he is stopped would only be a question of who is more powerful.
Now, suppose that a vegetarian thinks that eating animals is only 1/1000 as bad as killing Jews. If it’s okay to do otherwise evil things to save a Jew in Nazi Germany, then this vegetarian ought to be willing to do them to create 1001 vegetarians. So vegetarians should be shooting people right and left (or doing other inherently immoral things) if they think it advances vegetarianism enough.
Of course, it’s not possible for a vegetarian to be absolutely certain about vegetarianism, and once he accounts for his own uncertainty about vegetarianism in the calculation, the possibility that he is wrong and will have committed murder makes it no longer palatable to kill people to create vegetarians even if you’re creating lots and lots of vegetarians.
(And if he then reasons “okay, I shouldn’t kill to create 1001 vegetarians, but my uncertainty about vegetarianism is so low that it’s okay to kill to create a million vegetarians, that still doesn’t work because he is also uncertain about what his uncertainty is.)
Doing an (otherwise) mildly immoral thing to create many vegetarians is similar, except that of course the acceptable bound on his uncertainty about vegetarianism is higher—maybe high enough that he actually could justify it. But then this brings in other considerations—if he can mildly hurt other people for vegetarianism, then others will also be able to mildly hurt him for their own pet causes, for instance, and things may be overall better off if nobody is allowed to hurt others for their causes.