I do this. It works very well.

It is perhaps not what you might think though. The 12V auxiliary battery is fed by a DC:DC inverter from the 205V traction battery which in turn is charged by the engine.

I clip a small inverter to drive the house to the 12V battery and it runs until the 205V battery is discharged. The engine then starts and charges the 205V battery up again and cuts out automatically.

Just running the central heating boiler and freezer the engine runs for a few minutes, say 8-10 every hourish but it is only a 750W inverter I am using and the draw is minimal. I understand that the DC:DC inverter for the 12V battery is comfortable with 100A so no problem there.

The only trick is locking the car with the engine on. It needs two keys.

Have a Google around, look for 'Priups', there are a number of websites devoted to it so it is not out of the usual. I have read of people installing 205VDC to 230VAC inverters but I don't find that too attractive.