Anyone see anything wrong with using a 9v reg on the GND of the UCC283? I guess that would mean that the output could be temporarily switched to, which it see as, -6V, violating the pass elements maximum working VDS.
About your first design, that looks pretty awesome, if you're not overly worried about efficiency, a small charge pump with a 555 could be used so you could use an N-channel MOSFET as the pass element....although the transient reponse might suffer as the charge pump wouldn't be able to provide much current to cope with sudden changes in current through the MOSFET. I have seen some newer P_channel's with some very low on resistance...I'm sure your application could handle 11.9V when the battery is at 12V, with a 100mV drop at 2A, thats 50miliohms on resistance - doable with todays semiconductors.