RJJ tells TMZ Sports he's already met with Anderson, and has a group of investors prepared to hammer out the details ... and the only thing holding up the scrap is Dana giving Silva the green light to fight outside the UFC.

"Anderson wants it, I want it, so the only obstacle is Dana giving him the OK," Roy said.

Roy says that since Conor McGregor was allowed to have a one-off boxing match with Floyd Mayweather, Silva -- one of the sport's all-time greats -- should get to also.

FYI, we asked Silva if he'd take a non-UFC fight earlier this week ... and he didn't say no.