What if, when the grid goes out the inverter keeps powering the house (and disconnects from the grid).

Then when the grid comes back on, kill the output of inverter (which results in outage to the house), reconnect to grid, then power up the inverter again, which skirts all the sync issues.

The house will see a few second outage... but it will have been "on" for the entire grid outage, which I think is much preferable.