I would say it would be best to PM them directly. A few of my roleplays I've joined was because of a direct connection from the GM. @mentioning in a dead roleplay could come off as intrusive to the GM and possibly those who are invested in the previous roleplay so I would avoid that. @mentioning them in your roleplay would be better but I personally would be weirded out if it isn't someone I already know but I'm guessing not everyone would see it that way.