Here in Austria you have to keep 220 and 380V wiring seperate outside the panel. You'll usually have a 380V feed and individual 220 and 380 V circuits in individual conduits. I don't think a multiwire would be acceptable without using a subpanel, and in any case it'S very uncommon.
No, I don't think the Summenstromwandler inside an RCD would notice a broken neutral since all power going in returns through one or another phase. Only some of the appliances would get rather low or high voltages. I once saw a case like this, the fridge would only work if the electric range was on and the light bulbs were very bright...
