All events, including 'Bike To Work Day', benefit from advertising. T-shirts worn before, during and after the event are a big part of that. And most people who ride bicycles would wear a t-shirt as some part of their routine. So figure out how to make them a bigger part of the event by:
1) making them more affordable ($22.50 is ridiculous and if a fund raiser then an ineffective one)
2) making them part of the registration fee so everyone ends up with one (this is where you might do a little fund raising less painfully)
3) make them long sleeve to help with the morning or evening ride
4) make them up in all sizes (including XXL). What, you don't think big guys ride bikes?
5) make them less event-junky/ordinary and more attractive so people will actually want to wear them - the Bay Area is full of wonderful artists