Closing off the engine air bleed and opening the cabin bleed would drop the air pressure and evacuate the majority of the contaminated air allowing the air masks to drop and become the majority of the O2 supply to passengers while cutting the toxins.
There should be smoke detection in the ventilation system to automatically close the supply doors so smoke cannot make it to the cabin from the affected engine.
Varig Flight 820 - what a nightmare.
If the fire is held back by lack of oxygen then everyone is dead already. It will have moved to primarily carbon monoxide production along with heavy, choking soot.