Which pH should I aim for when neutralizing restored/refinished antique surfaces?
If you are cleaning something that can be washed, you can neutralize the pH with water.  After using HCl on bricks, I would just hose them off.  The acid will become very dilute quickly.  pH just measures the concentration of hydrogen ions in a solution, so if you add a bunch of water (neutral pH) the pH will approach neutral.

I also don't think you need to worry about the pH of solids such as dried plaster.