"Does backfiring due to bov venting to atmosphere cause any damage to the engine?"
Yes it can.
Any time the engine allows unburned fuel to pass the combustion cycle without being burnt, the fuel can wash down the cylinder walls and dilude the oil and create more wear on the rings and cylinder walls.
Having a turbo, it can also damage bearing seals within the turbo, damage O2 sensors, catalytec converters, valve seals and even valves and seats if left for a long enough time.
The computer (ECU) has little to nothing to do with the bad air to fuel ratio given when the BOV vents into the atmosphere.
The reason that it runs rich on BOV operation is because the ECU has allready metered the air before the BOV opens and the Air flow meter can not detect air escaping the system after the meter (or entering the system for that matter) and that means the ECU has no way to know that it needs to correct for the lack of air entering the engine when the BOV is functioning.
I suppose you could make some sort of adjustment for it in a stand alone ECU that adjusts the Air to fuel ratio when RPM is suddenly dropped or perhaps put some kind of sensor at the BOV, but it's just not worth it.
Simply plumb the air back into the air intake system behind the air flow meter in front of the turbo's intake impeller and you will not have this problem.
The turbocharger needs a pressure release when the engine RPM drops off because the exhaust turbine of the turbocharger slows down due to lack of exhaust to propel it. This means that the positive pressure inbetween the intake impeller wheel of the turbo and the throttle body will fight it's way back into the turbocharger causing surging and overheating of the turbocharger. This is where venting the pressure takes place.
A BOV is one way to do it, but a diverter valve (like your stock setup) routes the pressurized air from behind the intake impeller wheel back in front of it to equalize the pressure surrounding the intake impeller wheel, relieving the positive pressure built up behind the wheel at RPM drop as needed.