Kurt Sutter revealed in a February episode of his WTF Sutter podcast that he wants Marilyn Manson to provide some music for the final season. That podcast episode also revealed that Season 7 may be extended past its normal 13-episode length, while addressing that he is, "really trying to do my very best" to get all of the episodes down to 60 minutes apiece, since several Season 6 episodes were criticized for running well past the normally-allotted time.
As for Marilyn Manson "riding the dragon," that is a term used to describe heroin users, so it seems the rock star may portray a drug addict in the show.
Marilyn Manson has previously appeared on HBO's Eastbound & Down and played himself in an episode of Showtime's Californication. He also lent his voice to The Shadow in ABC's Once Upon a Time. On the big screen, the rocker has starred in Lost Highway, Jawbreaker, Party Monster and Wrong Cops.