I would expect the immediate reaction would be as you say. On the other hand, to someone afflicted by Liberal Mind Fog Disorder, victims are sacred. If the short said "I regret my abortion. Choose life," then LMFD could direct all the anxiety and hatred toward the last two words. But if the shirt simply says "I regret my abortion" and nothing else, I would expect that the cognitive dissonance would, for many people, not be resolvable without some cracks forming in the LMFD smoke screen.
"How dare that woman suggest that people should regret their abortions. I had my abortion and I don't regret it one bit. How dare she suggest that I should" (some time later, after succumbing mentally exhaustion ) "Why am I getting so worked up? I don't regret my abortion, well, except that I'm feeling pretty miserable. Why is that [bleep]ed woman so cool and collected. Doesn't she know that all this guilt-trip stuff hurts? How can she go through life bright and chipper with a shirt like that to constantly remind her? Why can't things be that easy for me?"
Not sure that would be anyone's actual thought process (probably not that quickly, at any rate), but I would expect that seeing such shirts from time to time would have that effect. Again, very important not to include extraneous messages. Just "I regret my abortion".
When the head of a local pro-life org relates her experience, she’s nothing like “bright and chipper”. Even if an abortion victim (the woman) chooses the repentence route instead of justification, it’s still a devastating hurt in her life.
I think you’ve got a good idea. The person agreeing to wear it, though, would have to be of strong resolve and faith. The answer to attacks would simply be “you should think about why you are so angry about this message”.