Hmm. You'll have to drill one out to accept the back-nut as they are drilled to accept the bolt. You're into mild bodgineering territory here, there's more to a stay bridge designed to accept a recessed nut than the diameter of the hole. They are often reinforced with a brazed in sleeve and/or have flats to accept the brake and nut.
I suspect that you'll find that adding the two concave washers will make the stay bridge too deep to accept your standard length back-nut. I have a few Campagnolo long back nuts in the workshop which should reach.
However, purpose made stay bridges have a small hole to receive the brake-centre-bolt and a larger hole to accept the recessed back-nut so another wee issue is that by drilling out the hole to accept the back-nut you'll have made the brake-centre-bolt hole too big for the brake-centre-bolt. There will therefore be a tendency for the brake to slip. You're relying heavily on the fit of the back-nut in the hole to keep the whole thing from moving downwards when you apply the rear brake.
What you require is a concave top-hat washer to simultaneously provide an interface between the flat of the brake and the curve of the stay bridge
and to sleeve up the centre bolt to fit the new stay bridge hole.
Unfortunately no such thing exists
