Is this ever going to be a thing?
Not sure what you fully mean. If you meant the costume locker working like it does in the cash shop, where we can try on costumes from gacha (or ones we don't own) piece-by-piece, then it's currently an unknown. I don't believe KR has this either.
What I mean is the ability to use images and packs from costumes which are in your costume locker without needing to have the full costume equipped or in your inventory.
Currently, it's kind of tedious to manage this if you wear outfits with pieces from different costume sets and/or if you like to swap outfits often.
As far as I know in KR, they haven't changed that. I don't believe a future branch update we'll get, which adds more customization options to portraits, doesn't add that in either. It is a pretty good idea though, admittedly I get annoyed by that too.