Since gearing up serves a similar purpose to leveling, how would you feel about getting a small amount of uncapped tomestones for every MSQ you do after hitting level cap? Like 5-20 tomestones per quest. I feel like that would not affect overall game balance and it would be a small bonus towards the gear grind that awaits. You could also choose to use those tomes on the materials and music rolls which are already available at the same vendor.