Divine casters don't know their spells (except for the ones that do).

As per the glossary:
known spell: A spell that an arcane spellcaster has learned and can prepare. For wizards, knowing a spell means having it in their spellbooks. For sorcerers and bards, knowing a spell means having selected it when acquiring new spells as a benefit of level advancement.
In the hypothetical where they do know their spells, however, I don't think acorn of far travel would necessarily work either. "As long as you carry the acorn [...] you are considered to be standing under that oak tree's canopy (and thus within an area of forested terrain)." In my estimation, you don't count as being in that location for any purpose other than the presence or absence of the tree. It's less "I'm effectively still in Rashemen" than "The tree is effectively following me around like Pikachu in Pokémon Yellow."