Am I right in thinking the various image adjustment options in Photoshop will ignore selection mask... i.e. these options apply to the whole image, or layer?

If that's right, and it is only filters that can be applied to a selection mask, then I guess you'd have to use your z-depth derived selection mask(s) to make cut-outs of each depth level you want, and split these across a number of layers... I think that's the approach I'd go with and work something out from there?

e.g. maybe you could create a cut out of your background depth level(s) and create an overlay layer with that cut-out... i.e. have the cut-out on an overlay layer set to "Darken" mode perhaps... if you see what I'm getting at?

You'd likely want to fade the edges of your "Darken" mode overlays... especially if elements in your scene receded into the background more continuously, and you didn't want a sort of parallax effect.

Hopefully my description of what I mean makes some sense?

Not tried this technique... but it's my best immediate suggestion ;)