1) If he wins the primary there's no way in hell the establishment of the republican party would do what you suggest. They care more about maintaining their long term viability than they do stopping Ron Paul. I would expect straight up voter fraud using their nifty electronic voting machines before I would expect some mythical third party run.
2) Donald Trump? Don't make me laugh. Have you seen the picture of him kissing drag queen Rudy Giuliani? And no serious republican politician would take the bait and run as a sacrificial lamb just to throw the election to Obama.
3) If they did pull an idiotic stunt like that then they would legitimize all of the third party movements that the two part duopoly is so afraid of. And they would have to change ballot access rules to pull this off, making it easier for 3rd party runs in the future. In other words this would hurt them more than it would help them. Throw me in that briar patch!
4) Why even talk about such things when we haven't even won a primary yet? Trying to cross bridges before you even know they exist, let alone get to them, just distracts you from the journey.