if we would make our tallis katan in a way that can be worn as a shirt, without wearing another shirt underneath, than this whole thing would not be an issue. But we don’t, and i doubt that that is happening. Maybe if some very big machmirim see this thread, they will start to do this. But very big machmirim usually don’t go on the internet.
Maybe we should go around to rabbonim, getting signatures, like some people do for some of the wierdest issues. And i guess that this would also fall under that category. Stop wearing regular shirts, and wear tzitzis shirts.
But its amazing that this is so unheard of, since there is a well known limud from shatnez, and like someone pointed out, that limud is for everything, according to tosfos, and we don’t know of anyone who argues. But we have never heard of anyone who makes a tzitzis shirt and wears it, like would be required for shatnez.
If anything, this could be a reason why not to wear a tallis katan, since it could be a bracha levatalah. Which is really what this entire thread is about in the first place.