You lost me at "at large positions determined by BCS rankings."
If it's not a good enough system to pick two teams, it's not good enough to pick any number of playoff teams either. Period.
(Scratched out oops paragraph that was the result of me misreading the OP's last paragraph.)
But what's the point of having national semifinals and calling it the "Sugar Bowl"? That's not "maintaining the bowl structure", it's just a weak, watered-down way to claim to keep the traditionalists happy. Nobody would proudly display a Sugar Bowl trophy if they lost the next game.
And, I would add, fans aren't going to travel to more than one neutral-site game. Witness BC fans last year, who helped make the CS Bowl the best-attended one in over a decade, but didn't go to Jacksonville at all for the ACCCG.