Didn't John Kerry propose something like this?

I think he did. Dick Cheney or maybe Bush himself derided the idea of turning Iraq over to UN or multinational forces. Remember how they all laughed at Kerry's use of "nuance" as in a "nuanced plan"? Maybe they were just laughing at the idea of a plan.

From a former Bush State Department Employee writing in today's Washington Post under the headline of A Brokered Peace - U.N. Mediation Is the Best Hope for a Political Settlement in Iraq:

Many ideas will need to be tested to find a political process that works. Here's one concept: The Baghdad conference participants could call on U.N. Secretary General Ban Ki Moon to propose a process leading to a political settlement that foreign ministers could endorse. Iraqis would be at the center of the process under U.N. mediation. Regional actors would enforce the mandate to get peace (and stay out of the conflict themselves). A "contact group" would engage international parties but keep them out of legalistic nuances