The Adjustment brush has a density slider, or is that not what you mean?
You can do this in Lightroom with the Local Adjustment "Amount" slider but I don't see an equivalent slider in ACR. Maybe someone knows of a hidden trick.
Yes, I'm hoping there's a shortcut in ACR.
Paint it on at low density, then right-click the Overlay pin and you can duplicate the adjustment which has the effect of adding to the density.