The best way to fill that one would be with a kahler tremolo
Nah, all joking aside, I've seen some pretty creative ways for filling in a route on here. The one that seems to be most common is to make a 'plug' out of a similar grained piece of ash and fit that into the route. The only problems I've ever seen with that are the fact that the piece may still be visible if you look closely, and the new piece may expand and contract differently than the original piece of ash used for the body but I'm not entirely sure about that.
I'm sure people with more experience will chime in, but I think the best route would be to make up a plug the same exact size and fit it into the route.