think you assume too much when you conclude a playoff would work against teams not presently part of the BCS, in fact I think that's a ridiculous statement. A playoff would necessarily include them, or else it wouldn't be a fair selection of teams. You are making the assumption, incorrectly, that it would have to be done by rank, only.
I propose a number of criteria be utilized that would assure a non-BCS school would be represented. And I"ve already outlined how that could happen
The only way the "big six" would opt for a play off would be if it was done by rank. No way they would enter into an agreement that would allow the #5 team in the country to stay home while the #21 team goes for winning their conference. the "big six" controls the money because they are the ones who make the money.
A number of criteria could be used to assure a non-BCS school would be represented, but it wouldn't happen. The "big six" will always protect their money. If they think they aren't getting their cut, they will change the system. Like it or not those conferences control CFB. If they decide to take their ball and go home, the rest of the conferences are in trouble.