Gleb, first of all I think that Intentional Insights is a great idea and I support the cause! Unfortunately, I don't think I'll be supporting the cause by wearing t-shirts with slogans. In particular, InIn (as I understand it) is aimed at the broader audience, while all those t-shirts are basically in-jokes for people already in the LW space and baffling to anyone else.

I second ChristianKI's idea of incorporating graphic designs, maybe an "I changed my mind today!" shirt in the style of this famous one.

I would also suggest making the slogans intriguing and humorous instead of straightforward, so they can start conversations and be less embarrassing to wear. Off the top of my head:

• I was stupid yesterday instead of "Less wrong every day"
• Super agenty instead of "Living on purpose"
• Evidence or it doesn't happen instead of "Absence Of Evidence Is Evidence Of Absence"

None of the above are really good, but that's the direction I think the t-shirts should be going in.