I don’t see any real possibility to do this unless you know the appropriate people. Which period are you going ?
Generally, around 1 pm is quieter because all those horrible package tour groups are gone for lunch and you have only individual visitors. But of course, this is the hottest time of the day.
But also, you have to know that Angkor Wat complex is huge so the effect of crowding is not as bad as in other temples, like Ta Prohm. There is a big dilution effect in Angkor Wat.