Only really possible if Edward dies in the 1460s, and truthfully that opens up the possibility of a longer more successful Lancastrian recovery of the throne. If you have Elizabeth miscarry with Edward V (due to the panic of Edward having to flee to Burgundy say) and Edward die while in exile, you could get a very interesting scenario where George becoming the Yorkist heir while being allied to the Lancastrians which would almost definitely see him rebel but the question is do the Yorkists support him or do they support Richard?