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."
So, this would mean you never get sunburn, a Horizon Walker gets Hide+4(Competence), a Ranger 13 can use Camouflage anywhere, and Ranger 17 can HiPS anywhere. That does seem like a nicely toned down interpretation more commensurate with a 2nd level spell.
It still leaves Universal Spirit Magic granting an Archivist 1/Wizard 7/Hathran 10 the ability to cast 3 up-to-9th level Archivist spells per day, something I hadn't fully appreciated.