I think the best way to do it, is to provisionally log the hours either as PIC if you are PIC or as Copilot if you are SIC on that individual flight if engaged in multicrew operations. If the aircraft is certified for single pilot operations, that just implies that the aircraft can be operated this way (and that it is not a requirement to hold an ATP to act as PIC) - if any flightoperation decides to implement a multicrew operation, it does not make sense to penelise the guy doing his job in the right hand seat.
Another thing to remember is that the 1900 does not comply with FAR 25 performance certification requirements. JAA require 500 hrs on a FAR 25 certified aircraft to unfreeze an ATP. I just mention this on the sideline to add to the discussion. As SACAA seemingly want to go the JAA way, this is what might be in store in the future.