Actually, one of the ways to ameliorate the shortage of hulls is to run the SFO service about 6 hours late. This enables the inbound QF32 service from LHR - SIN to be turned around and sent to SFO. Naturally it can't be done for too long given the need for maintenance and OSIP components to be checked.
I know about this one - my SFO trips were delayed when we clouted the blast fence in JFK as well as this time.