Agreed, regardless of the name or dominant management team the combined airline would not be welcome to remain in Star Alliance. It would probably take US Airways 18+ months to extricate itself from its Star Alliance contracts, which suggests there wouldn't be meaningful status changes until 2015.
American Express already provides OneWorld status in some other markets through Cathay Pacific. If they were not able to reach an arrangement with the new American Airlines they might be able to extend the CX benefit to USA Centurion cardholders.
Note, however, that CO exited the ST alliance quite suddenly and quickly.
They moved to *A well before the merger with UA completed (some might opine it was before the merger even began its initial practical implementation).
Not that one can necessarily draw conclusions from the CO/UA case, as many things aren't similar (e.g. neither US and AA are MR partners, but both are Plat/Cent lounge partners - in CO/UA's case the former was a full partner for both and the latter was an influential enemy of both).
A purely speculative gamble on my part would be that US leaves Amex altogether (indeed leaves the aviation world and is merged into "new AA", like CO left the aviation world and was merged into "new UA") and that Cent finds an elite-partner replacement such as CX for the USA, giving Cents OW status and enabling the continuation of AA lounge status (also in OW) but not MR status.
UA/Chase would stymie any attempt to add *A status once US is gone, so the only addition to the ST of Delta would be an alternate OW status with a foreign-based airline...
That's where my chip would go on the table.