At last year’s Loscon I saw Larry Niven wearing his 2005 Marcon t-shirt. On the back it listed maybe a dozen different guests with a category/definer for each. I know nothing about contemporary Marcons beyond what it said on this shirt, but I wanted to report that my immediate reaction was “These guys look like they’re making a sophisticated effort to reach out to as many segments of sf fandom as they can find.”
Fans generally acquire con t-shirts at the con, so impressive as I found this one to be it probably wasn’t a component of Marcon’s pre-convention advertising.
A lot of committees start talking up their program participants well in advance, list them on websites, send e-mails to lists, etc. to attract interest. Maybe shirts could be another part of that strategy. That takes a certain amount of money, so it might be more practical for a large regional or a Worldcon. Think of a lovely pre-con shirt to be worn by Worldcon committee, given to patron friends, available for general sale, etc., as one more creative to get the attention of fans at other cons and start them thinking about attending yours.