Looks like I was completely off in my interpretation of that illustration by Jeff Dee from Expedition to the Barrier Peaks. If you take a look at the following image, also from the same module, you'll recognize many of the same party members from the previous piece, like the bare-legged female fighter, the cleric, and the wizard. Also among them is a fighter whose helmet has a plume.
It's not the greatest scan of the image, unfortunately, but I think it pretty well clinches the feathered helmet hypothesis as the correct one. Ah, well.