Rumor or not it would be a great idea. KTM and Red Bull have deep pockets and quite frankly if you want to win the best rider will help your odds. How much money does a guy need to collect before he-she is willing to give up a huge bragging right. I believe every rider in the paddock would want to clean up on an unproven machine for the egomaniacal pleasure alone. PS Philip Island was a cracking race, the best I can remember.
KTM has said they will not replace there riders, they believe they have the right stuff. If Marc wanted to ride for them I bet they would find a bike available.