As absurd as it sounds I don't believe Dragon would save Luffy if he was going to be executed at Marineford. Reason 1 The Revolutionaries aren't ready to take the fight to the World Government. Dragon isn't a man that would waste his Military forces just to save his son. If Dragon brings his forces to Marineford then the World Government will capture there territory and everything they've done since the start will have been useless. Reason 2 Dragon is too important. If he dies at Marineford then the Revolutionaries will have no one to lead them and fall apart. Dragon wont risk going there and dieing which is likely since the Admirals will do anything to take his head. It will be a short term pain (Luffy) for a long term gain (overthrowing the World Government). Reason 3 He doesn't like to interfere with Luffy's life. Sure, he saved him at Loguetown but that's a piece of cake compared to saving someone from Marineford. He knew Luffy would go to Marineford to save ace and didn't interfere and I don't think he would if Luffy was the one being executed.