I've tried organizing in Buffalo, but I think there's just no real call for it in Upstate as there are public gardens in open lots fapproved by the city, it's mostly gardening clubs that take care of public areas in the cities (Syracuse, Utica, Rochester)
Mainly it's organized, and not very guerilla....
http://www.geneseonygardenclub.org/bonsai-upstateny.org
Brockport Garden Club
Gwyn Parkhurst
585/637-2822
ahsregion4.org/flds.html
http://www.gardencentral.org/fgcnyMen?s Garden Club of Syracuse meets at 7:30 pm the 3rd Thursday of each month, First English Lutheran Church, 501 James St, Syracuse, NY. Rear Entrance. Non-denominational; all welcome, come as you are, free. 315/420-6369, (and the Lord said, let there be flowers...)