I was just reading this article about "Operation Prom" up in Westchester County, which gives free prom dresses (or BM dresses) and tuxes to needy teens.
http://www.msnbc.msn.com/id/36107802/ns/local_news-new_york_ny/It didn't give a website for the organization, but if you are looking to donate old BM dresses or formal dresses, maybe this is a resource you'd like to explore.