First, I don't think a shirt is going to help much. For some people it will be too eager, a little weird even. For others, they aren't going to read it. But let's say someone reads it, believes it, takes it at face value, and wants to chat with you. Are you expecting them to come up to you and start a conversation? To do the work?
Second, I think you will find you are in a vanishingly small minority. I don't know anyone who likes talking to strangers on planes. I am on my technology (or asleep) during my long flights because I would prefer to do that than talk to anyone. I am sure people exist who enjoy a good chat -- I have seen strangers talk on cruises and ferries and tours -- but it seems to be a "pass the time" thing for the trip and not the start of a beautiful friendship.
But, should you want to make a friend, and you're prepared to accept that perhaps nobody on your flight shares that want, you are going to have to be the one to do the work. That's going to mean smiling at people, greeting people, and saying things to people that our society uses to start conversations. "I love your earrings!" "Oh, I was thinking of seeing the movie of that book you're reading, is it good?" "Did I see you working this same flight last week?" You need to be really sensitive to people not wanting to talk, and ready to leave them alone in that case, while at the same time being bubbly, outgoing, and warm. [Be especially cautious with flight attendants, who have to be friendly to everyone even if they do not want to do so, and with a seatmate or anyone else who may feel trapped and unable to leave if your attention is unwanted.] If the person is receptive at all, you can introduce yourself. You can say you fly this route a lot. You can ask them about themselves. It may take several shared flights before you feel a friendship is developing, or perhaps you'll click with someone right away.
It might also help to have next steps available. For example, if you take a sightseeing day at the start of each trip for jetlag, you could invite your new friend to stroll some famous park with you. If you already have a cab to the city booked, you could offer to share it with the person. You can give them tips about great restaurants or bars or stores that you've found in this destination. Share your enjoyment of this destination with your fellow traveller. Or, if they are going home to the place you're visiting, maybe let them share their knowledge with you.
All of this is work. But once you're over 20, friendships grow from work. Not from Tshirts.
One other thing: you mention only wanting platonic friendships. Some people who are receptive to your greetings and conversation starters may be hoping for something non-platonic. Be alert for signs of that so you can make your intentions clear early on, and don't escalate too quickly to "spending time alone in private places" with someone you've just met.