Not sure that adding anything on top now will improve things. It's all down to relative refractive indexes of teh decal sheet and the finish. Whereas nitro or TruOil seem to have a similar refractive index to most decal papers, Wudtone would seem to be different enough to show up the decal sheet at the wrong angle.

In that instance, adding something on top won't improve anything and may make matters slightly worse.

If it's that important, I'd bit the bullet, sand back the face of the headstock to bare wood, and re-do it in something like Tru-Oil, before and after the decal goes on. I'm assuming here that Tru-Oil does work well with decals if used both under and on top as I haven't seen anything on the forum to the contrary, but I can't speak from personal experience.