It would be nice to know if it is a 24hr access to the outdoor enclosure as getting them back in could be fun, which may limit the scatter feeds outside to ensure they can secure the macaques.
I also wonder if such fears will change over time for animals? Long term research would be good on this.
I imagine they might not have 24 hour access outdoors for security reasons? You have a point about the number of scatter feeds possible in that case. I don't know what % of their time the Sulawesis currently spend outdoors(I have seen them outside at Chester in the past and they are certainly a good display then) and I am only going on ChesterZooFan's observations there.
Re overcoming fear/reluctance of the open over time(referring generally here to any forest Primate species). I think in the zoo situation its more an ingrained preference for the covered situations which is very hard to alter and I think it would take very many generations to significantly change their natural behaviour patterns. But for display purposes, effective management e.g. auxilliary feeding, can certainly help.