Marc Ambinder makes the relevant point about an Obama-Hagel ticket: "There's a much simpler way for Obama to reap the benefits of having Hagel endorse him -- and that is to have Hagel endorse him."
Right. Any prospective Obama-Hagel partnership would be premised on shared ideas about foreign policy, so the natural thing would be for Obama to tap Hagel for some kind of foreign policy job. I don't think it would be a bad idea to have an internationalist Republican representing us at the UN, say. But ultimately Hagel just needs to decide whether or not thinks his old friend John McCain's foreign policy views are so unsound that he wants to endorse his Democratic rival.