So it's possible to use front face accessory on Spiral Round Helm, but it's not possible on a costume Round helm??? It doesn't make any sense, they look the same, both are exposing the face, i dont get it, maybe round helm is not the most popular helm but since its always in prize boxes i think Spiral knights should fix this problem and let us use front face accessories in costumes round helms, a game face isn't going to hurt anyone!
Make's sense, how about submitting this as a bug. Maybe you will get more attention form the devs instead of the suggestion forums.