[mindslight]

For a generator, yes.

Is it currently possible to do this with a battery setup, for which its normal state is to feed power to the grid with anti-islanding?

Either you'd need two power connections to the panel - one for the every day anti-islanded backfeed, and then a second with the physical lockout to a different inverter output that operates without the grid.

Or the lockout on the main breaker would need to control a logic-level switch that told the inverter to disable anti-islanding, for power flowing through a separate non-locked-out breaker. This would seem like a better solution, but the inverter/battery manufacturer would have to design for it and get NRTL approval.