Only if the t-shirt is alive. They've disallowed alive genetically modified organisms, not their products or derivatives.
Technically still not allowed:
>Projects cannot resell items or offer rewards not produced by the project or its creator
I'm pretty sure that that doesn't require every input used in creating the reward to be created by the project or its creators, so a T-Shirt created with (dead) GMO cotton which was produced by (e.g., screen printed under contract for) the project would still be allowed, even if neither the cotton nor the ink was produced by the project.
So kill the plant and that's ok, but the living plant isn't?