Couldn't you test it by either isolating an existing region and try to blackstart that particular region, intentionally out of phase with the main grid, and then try to synchronize and connect.
Black-start-region and join-regions should be enough to get a whole grid going?
If you can handle the consumer load in the region without local plants, maybe? That would still only be a 90% test because you can't completely blackout a region for testing purposes, consumers would get angry.
Black-start-region and join-regions should be enough to get a whole grid going?