I haven't tried this specific combination but there's an option in the outfit menu to show whatever you actually have equipped in that slot instead of an outfit piece.
I think if you select that (note that for hats this is separate to the hide hat option) and then equip the hat you want via the collections menu it should show.
PC EU player | She/her/hers | PAWS (Positively Against Wrip-off Stuff) - Say No to Crown Crates!
"Remember in this game we call life that no one said it's fair"
sorry i have to specify - i mean at the outfit stations so i can look at the combo before comitting
NO. I just tried to put the new Light Tsaesci Hat on my Adventurer's Costume and it FAILS. There is no combination of hides and shows that will let me have that combo. EXTREMELY disappointing I was looking forward to that look. Also the light Hat looks REALLY small on my Khajit so double disappointed.
Oh well you can't have it all I supposed, but just wanted to clear that up, basically what the first reply said, maybe on some but not if the head is part of the Costume. Although I can put a Costume Hat on top of the Costume, I wish they would change this so the Outfit system can override all or parts of the Costumes. Polymorphs are a whole 'nother matter.
Polymorph weapons (if they ever become applicable)
Outfit weapons
Polymorph base appearance
Collectible Hat/Helm
Costume base appearance
Outfit armor
Slotted base gear
Outfit armor won't override costumes or polymorphs because neither of those categories are required to design around all seven basic armor slots. So they function more as two piece skins; costumes with a head gear replacement option and polymorphs with the occasional head gear toggle on/off.
A possible workaround is to do informal polling on which standalone motif hats/helms would be most popular. That would give ZOS some numbers to work with, perhaps with something like a fan favorite bundle.