On the JP servers, some outfits were released as basewear as well.

E.g. Zelsius This is the release post about it. https://www.bumped.org/psublog/pso2-sg-scratch-arks-battlewear-selection/

Are there plans to release the same basewear versions here? This is significant to me because I rolled a CAST this time, and it didn't occur to me that many of the outfits I enjoyed cannot be worn by them.

Agreed, I think all the outfits should be layered wear so people can mix and match (I'm still not sure how it was legal for them to do in Japan though considering they have gacha/lootbox laws that forbid multi-part items, what they call "kompu gacha"). Also for future reference, this sort of topic goes here https://forum.pso2.com/category/61/in-game-suggestions