I doubt the league would allow this because it would be a conflict of interest on the part of Jay-Z. Because they already have a business relationship, the league wont allow them to have two relationships going.
Jay-z could represent Lebron as long as he wasnt playing for the Nets. The only way it would be a conflict of interest is if he was on both sides of the contract (ie. jay was his manager and signed him to the nets). However, the league would prob frown upon Jay managing anyone because he owns the team.