What if you painted the "N" in white directly onto the insert, then placed the decal over it? Maybe you could use another insert decal as a stencil. Cut the "white" part out of the "N" place it on the insert and fill it in with white paint and remove the decal stencil. When the paint is dry apply the new insert decal on top of it. You would probably want to use decal setting solution so the decal conforms to any irregularities in the insert. I know that's probably not the easiest way but it may be a good option since your local printers can't print white.