In Himeko Sutori, when I zoom the camera in and out, I change the DOF settings directly, without the OverridePostProcessSettings.
WorldInfo.DefaultPostProcessSettings.DOF_FocusDistance = ...
WorldInfo.DefaultPostProcessSettings.DOF_FocusInnerRadius = ...
This works just fine for me, and nothing undoes the settings that I change manually. I don’t know the downsides to how I do it. Maybe this approach could cause problems if I used more post process volumes, or if my game were multiplayer.
Maybe I should start doing as @ says and start trying to access PlayerPostProcess instead.