So, I managed to solve it, but it still really mysterious the whole thing.
Turns out, the hat and gloves from the picture are part of the hak pak version of some of these armors (for some reason I could no longer find them in the override folder, but…well, it’s pretty bonkers the whole thing): MP Armors | The Neverwinter Vault
I was lucky enough to find another picture with clues and text from where it was from (by doing some detective work looking through all the stuff, located in a separate folder on another harddrive, I’ve downloaded from the vault throughout the years) .Turns out they were to be found in the hak pak file: aeris3_mparm6 from the MP Armors. How in the name of the all the forgotten realms gods I was able to have that hat and gloves in the toolset WITHOUT the hak pak in the first place remains a total mystery. But now, after ascociating the hak pak with the module/campaign, they show up both in the toolset and ingame.
EDIT: What could have happened, now that I think about it, is that I did some loading of an old module of mine, and the new, back and forth to check some info I needed about my custom world, and I think I use that hak pak in my old module. Sometimes the toolset doesn’t update when you switch things around like that, and that may for some odd reason have made the hat visible in the toolset, although the hak pak wasn’t ascociated with my new module, but when running the new module of course the hat wasn’t there.