Don't forget there's already a 'specialty' woods adjacency bonus for Norway's Holy Site with a Stave Church in it.
As for 'Druidic Lore' woods bonus, the historical background/prototype for it is not Celtic or Gallic at all, but the Roman Propaganda concerning Druids, which is not the same thing. The ancient northern European reverence seems to have been for the Oldest and Largest Living Things around, which were Oak Trees, and that shows up historically for certain in the Scandinavian Paganism and possibly, archeologically, in pre-Celtic religions of northern Europe. So, if the game was to have a 'Woods Bonus' for religion, it would have to be strictly limited to Old Growth, Never Chopped Forest tiles only - which wouldn't be such a bad idea, since it would provide, possibly, a small 'brake' on the Magnus Chop Distortion in the game now...