While it is technically possible to retain IR3 under Elec Emergency Configuration by setting ATT HDG to FO3, the Pitot and Static Probes of System 3 (and 2 as well) will loose the de-icing capability. Thus while flying in icing conditions one might end up in an Unreliable Speed Scenario with Smoke on the Flight Deck and serious time pressure. No thanks.
So the only way for Airbus to offer use of this 'dirty little trick' to the crew would be to ensure in an early state of a complicated QRH procedure an during a demanding situation that absolutely no icing conditions are expectet from PPOS all the way until touchdown at the diversion airfield.
The Smoke-Procedure is already complicated enough and lot's of procedural mistakes happen without another biggie option as described above.