Found this little entry that mentioned J at the Mr. Showbiz site from a report about the Sundance Film Festival (I don’t think its anything new. I remember reading that J was going to appear as a member of a band in an Alison Anders film along with members of Redd Kross):

Four-time Sundance vet Allison Anders won thunderous applause after her extremely personal film Things Behind the Sun premiered Wednesday. It starts as an Almost Famous-like tale of self-destructive musician Sherry (Kim Dickens) being profiled by rock journalist Owen (Gabriel Man). Any other similarities to Cameron Crowe’s nostalgic rock flick, however, quickly disappear as the film flashes back to Sherry’s harrowing rape — an experience that eventually shapes her as a musician but shatters her personal life. After the screening, Anders revealed just how personal the film was: Not only had she also been raped as a teen, but she had also returned to film in the same house (whose address was 7666, scarily enough) where the assault took place. The music-savvy Anders secured the services of Sonic Youth to create Sherry’s songs in the film, and one of her band members is played by Dinosaur Jr.’s J. Mascis.