Just throwing some random ideas out there.
"Aikido : Reach out and grab someone."
"Peace through greater throwing power."
"Who needs a gun (knife,weapon), when you can hit someone with the Planet?
"Aikido: Size doesn't matter. (...but it makes things more interesting!)
(Here's a tacky one, but I think it's funny.
On the front of the T-shirt, where the "pocket" space would be, it would read:
... on the back of the shirt is would say: "Grab my wrist. I dare you."
I'll keep working on this. I'm sure there are some good ideas. I know I've had better ones before this.