As far as I can tell, actual displacement. Capture of the Shader Settings:
https://www.screencast.com/t/9A1wVrFAtT1Additive (and a bit subtractive) sculpting is exactly what I was doing. I turned my brushes down to low-flow, set the spacing to nil, and then varied the stroke opacity depending upon how much I was painting. It worked very well with fill layers and masks for laying on effects. First I would put down general large shapes, and then refine them, generally sticking with additive sculpting until it came to putting in finer details. I'm not practiced with sculpting, but this worked for me.
Unfortunately, no matter what shader I threw on the mesh, it was incredibly difficult to see where I was sculpting without constantly pausing and rotating the model and/or environment. It was very flat. On the bright side, I did learn how to do this much more quickly by remapping the buttons on my tablet stylus and keeping my keyboard beside it, but it still would have been a real boon to be able to see the defined shape as I worked on it.
I don't think it helped that the normal maps appeared to ignore the painted displacement. Putting a "height to normal" layer didn't do anything, either, because I was painting in the actual displacement channel and leaving the height channel for smaller details baked into the substances I applied after sculpting was finished.
I really did love sculpting in Painter, just wish it'd be easier to see!