I could be wrong about this, but I think only the area actually being plated counts as surface area.
If you plug the pipes the inside is not plated or exposed to the solution so it would not count as surface area. If you plate the inside of the pipe also though then it would be surface area.
If I am incorrect please let me know
I beleave also if you are plating someting like valve covers or air cleaners, if you paint the insides so they are not conductive to electric they will not plate, so you would not count the inside as surface area either. Just the outside would be plated, so just the outside counts.
Chromo