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I don't suppose at the very least costumes could have their head piece separated? At least then I could use Mannimarco's costume's head piece on its own.
Unfortunately, you can't separate headpieces off of their costumes. They did that once, where the Hakoshae Festival Attire costume, instead of having a head adornment part built into it, got a circlet bundled together with it - but they made the Hakoshae Festival Laurel a separate Adornment entirely so that was the exception.
You can wear regular Adornment circlets with any Outfit and Costume though. There are quite a few circlet Adornments (red diamond, leather, psijic, pact, etc)! And you can also wear Hats together with Costumes. There aren't many circlet type hats, but the Ayleid Royal Crown has a crown/circlet look to it.