Ok, I think we're over evaluating these.
It ain't gonna be a problem!
If he polishes/and seals the outer portion of his plugs with clear paint,,,, it will be fine.
If he polishes/and doesn't seal the outer portion of his plugs with clear paint,,,, he'll have to polish it regularly to keep it shiny..
Explanation:
I have a 1912 Model T Ford. (The early T's are generally referred to as a Brass T because of all the brass they have on them).
Mine has brass headlights, radiator, windshield frame, running lights, tail light, steering when, horn, hub caps, etc etc etc...
It' is now 102 years old and the brass (which is mostly thin brass sheet) has not corroded away.
The brass corrodes, not the steel it is attached to...
Now, I admit it's a real PITA polishing all of that brass when we take it out,,, but it ain't a big corrosion problem for the overall car...
Ride safe, Ted