Taking your scenario, 7 twelve team conferences presents 84 BCS teams. Counting the current transition teams, there are 119 1-A teams. Also, it is understood, the NCAA may just allow one division A, but keep playoffs for those at 63 scholarships or so.
Assuming, hypothetically, the PAC 10 expands with 2 teams, say Utah and BYU, and reaches twelve.
Asumming the Big10 adds one, say Notre Dame, and reaches 12.
Assumming, using your proposal, the Big East expands with the BE eight plus Sou. Miss, Memphis, Marshall, and East Carolina (I realize you had 'Nova, but will use Marshall here since they get frequent mention and are established).
The ACC, SEC, and Big 12 remain the same.
The traditional BCS 6 would now offer up 72 teams. With these moves, 12 more teams will be needed for the 7th conference. The MWC lost two (now at 7), C-USA having lost four to the BE will now be at 8. The SunBelt, with the two transitionals, remain at 8, the MAC remains untouched, remaining at 12, and the WAC was untouched, remaining at 9 assuming SJSU stays.
So, who are the candidates for the additional (7th) BCS conference? The two who were previously argueably the strongest, C-USA and the MWC, dropped in members, neither having twelve in the first place.
The competition would be on for the 7th conference spot among the following:
(a) The seven teams from the MWC consolidate with 3/4 of the current WAC. That offers up New Mexico, San Diego State, Wyoming, CSU, Air Force, TCU, and UNLV who consolidate with Boise State, Fresno State, Nevada, Hawaii, and Utah State. (Hypothetically, say LA Tech, NMSU, and SJSU are the ones left out)
(b) C-USA's remaining eight: UAB, UCF, Tulane, SMU, Tulsa, Rice, Houston, and UTEP, expand with LA Tech, North Texas, NMSU, and Arkansas State.
(c) The MAC says they want a part of this. They will still be 12, but three: Akron, Buffalo, and Eastern Michigan don't meet the critieria; but, the MAC replaces them with Army, Navy, and Temple.
(d) That leaves the SunBelt: They can't get 12, with only LA-L, LA-M, MTSU, FAU, FIU, and Troy remaining. They will seek to find some more southern upgrades currently at 1-AA.
Therefore, the SunBelt could be ruled out as the 7th BCS conference. The MAC, keeping 12, could argue that Northern Illinois, BGSU, Miami of Ohio, and Toledo are pretty good teams, and can compete.
The strongest arguements would be between C-USA (revised) and the consolidated and revised MWC/WAC.
(a) The arguement as to who would be the strongest 7th conference would be more profound.
(b) Both the revised C-USA and MWC/WAC would have teams that could compete at the midlevel of the NBE and one or two others.
(c) Colleges and Universities change. Anything permanent amounts to exclusion and almost anti-trust like. Suppose, for example, Northern Illinois or Louisiana-L emerge over time to become far more deserving of access compared to BYU, TCU, or Rutgers.
(d) Even by demographics or politics or another reason or motive, the composition of even the BCS conferences could further change.
(e) Suppose Montana, GA Southern, UMass, WKU, or Southern Ill., want to go big time? Theoretically, a reasonable avenue needs to be available.
Fundamentally, the BCS needs revamped, and "left outs" need more than just one spot to vie for along with major conference runner-up's unless major conferences are still limited to one BCS spot each based on championship games. If the remaining current I-A teams are competing for one spot among 36 others, while BCS schools are at one in twelve for the competition, this design is inheriently unfair, and worst than the current system. There is less flexibility, and less opportunity to recognize characteristic changes among the mass of universities. If the 1-A and 1-AA distinction is removed, which appears very possible, then the 36 or so number will grow, or even more schools may be regulated in the category for the 16 playoff group (or fall somewhere inbetween?). For some, though, that would not be a bad idea; they would need to drop their scholarship counts. These ranks would swell and post-season opportunities would become even more acute.