I've got a 2006, and looked into it a while ago, and unfortunately it isn't a simple change... I don't know if the 2012 is different or not, but for 06-10:
The seats are both different in the second row, to allow the mounting/attaching bracket on the passenger side as well. Additionally, you have to get the seat-belt tensioner mounted to the 'roof' of the car - I think the mounting holes would still be under the headliner, just a matter of locating.
Finally, unless the connection methodology changed, I heard a major reason the seat wasn't offered here is because of crash-test rules. As I heard it, the rule required that the seat be mounted directly to the frame of the car, being connected to another seat in a temporary/removable fashion didn't pass muster... don't recall exactly how that one worked, as now the Toyota Sienna has a similar configuration. Don't know if it's because it connected to BOTH movable chairs, one on each side, Sienna only connects to passenger side seat...
Think both middle-row seats were near $1k for parts, plus installation if you don't do it yourself, but it's still not legal... I'd feel comfortable from an engineering standpoint, but if you were ever in an accident, the insurance company would probably not cover anything due to 'tampering'