Videographer Rob Parsell hosted a screening for the trailer of his forthcoming documentary about famed Bad Brains–singer H.R. last night at DC9, but didn’t know until late that he would be joined by a special guest: old Human Rights himself. The documentary, which is still in production, covers the story of H.R. (and by extension, his legendary hardcore fusion band Bad Brains) from 1984 ’til the present. “He’s such an enigmatic person. A legend,” says…