You do not HAVE to pay FICA if you are employed by USAB. If you WANT to pay, you MAY. Once the US basing goes "onshore" you'll be working for a US company (i.e. Cathay Pacific USA, Inc., which employs the ground personnel and the flight attendants based in SFO) and will be treated as any other American employee.
I know of no legal way to keep the 15.5% not subject to US taxes. It would need to be placed in a US-recognized retirement fund (i.e. 401(k) or SEP-IRA), of which USAB has none. Once again, when the US basing goes "onshore," this will change.