I think the trick is going to be either: 1) finding hashtags with enough people desiring them so that you can order in bulk OR 2) finding a production method that is cheap/efficient enough that you can print 1 or 2 shirts without breaking the bank

I wonder if you could do something like threadless did, where people vote for the "best hashtag" with their wallet, saying they would buy one if you produced the lot.

Not bad for a weekend project though! Props to you!