Gin, you've got it completely backwards. The tshirts are based on the comics, and not the other way around. Ever.
I mean, honestly! Now we're saying the comic is a "merchandising vehicle" because some of my shirts were based on one-off characters or gags? That doesn't even make any sense! If all I cared about was selling t-shirts, then Dr Crab and the Overkill Robot and Skull Panda and "I'm Communism" wouldn't have disappeared... they would reappear constantly, to keep spurring new interest in their shirt designs!
I know we are all jaded and cynical these days, but please, let's not get too conspiratorial! Comics with stand-alone gags and iconic-looking characters are the ones that translate best into stand-alone, iconic joke t-shirts -- that's all there is to it. Not a trick, not a conspiracy, Scout's honour.
You are hearing me talk.