To give you something else to chew on while you contemplate biking "critical mass" type events: At my university in NYC there was a Professor who led a midnight bike ride around the city and lectured on history while he did it - these events were always shockingly well attended. It had a lot to do with the relative star power of the professor. Though you might not be able to get the same in every city you want to take "Local Lore" to, I think you could reach out to local celebrities to get them involved. They might appreciate the chance to give back to the community while getting free publicity, and your target audience might have folks who are not naturally drawn to the idea of local lore might love to ride around with their favorite local band for a day and hear about their take on the community. Pairing the the local celeb(s) with guides who have deeper expertise but lack the star power might work well.
In case it hasn't been done already, it would probably be useful to join forces with Greek systems on campuses (where they already exist) as well. Most Fraternities and Sororities already have service and community involvement requirements to keep their charters, and they have significant resources that they can leverage too. Their payoff could be some good PR and a sense of being ahead of their competitors - and students and alums always want to see their communities doing well as opposed to the opposite.
I think you can also think a little bit bigger on customer base too. Political campaigns at every level (local, state, Federal), are always looking for short term space - and they bring a ton of vibrancy with them (though by nature they shut down when the election is over). Big charities too (I am thinking Race for the Cure, etc., could probably use short term space for prep weeks in advance of their events and for execution on gamedays. These types of organizations could really benefit from consitent, long-term relationships with an outfit like Zipspaces.