I've seen wood filler used used before, both good and bad, and when used properly it is almost invisible. I can not see it working long term along the edge of a lock mortise for example. The lock could be removed several times in its lifetime, that wood filler will break out. In such case, a piece of wood glued in or patch from acraglass tinted to match will hold in better. I had the bad luck of purchasing a rifle where either the builder or seller made a "repair" using wood filler, after shooting the piece for a bit, I pulled the barrel to find a hidden crack in lock region and wood filler being used as bedding for the barrel with wood missing under it and behind it. This was not a case where I could go back to ask for my money back, lesson learned. As a cabinet maker for 20+years,I've used my share of wood filler, there are uses for it, but still recommend other methods for rifle construction due to long term adhesion.