While I think this is a terrific idea, I couldn't help but think that these artists might prefer space indoors, and might need electricity, a flat surface to work on, and so forth. Where might one find such a space? How about in the street-level empty retail spaces that grace many of the new condo and apartment developments around the hill?
For example, John Court, at 15th and John, has a huge empty retail space -- one that could either be a restaurant or be subdivided into three smaller shops; they are probably hoping to get a tanning salon, a real estate agent, and a check-cashing spot -- that could easily accommodate a small theater troupe, dance crew, guerilla projection team, etc.
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The major obstacle to such an arrangement, and a recurring theme around here, would probably be liability insurance. Other than that, I can not imagine why a property owner would not be open to leasing space out temporarily, especially to the arts. In such a situation, everyone wins: the artists have a place to rehearse, the owner gets good PR and can bring in a few bucks without substantial risk, and the street becomes that much more lively during rehearsals.
As an exploratory effort, I'm going to put together a google map showing buildings with this sort of dead street level space (wanna help? email me). I have asked the emailer to round up a list of artists in need of such space, and hopefully we can find a way to help them make connections with the owners. Also, if you have any connections at companies like CBRE, I could use all the inside help I can get.