Hi,
I just came across this idea... A "Write-It-Yourself" T-Shirt!
Essentially, the T-shirt has part of the message, and you write in the rest...
Although this started from a particular "geek" subculture - I can see how this idea could be adapted to all sorts of things...
Read more here -
http://www.thinkgeek.com/tshirts/generic/9505
Cheers,
Dien