Contract Clauses

So with my hopes of Winston joining the red & pewter next year, I keep hearing people mention baseball as a potential distraction. Can't they just add a clause in his contract? "The player shall not engage .... MLB, any other organized league, unless otherwise authorized prior..." etc etc. Is this something the NFLPA wouldn't allow? Has to be something to it because I couldn't have just solved this, while drinking and surfing the internet.