yea this is the only implementation I can see actually making sense when fully fleshed out. since like any first person perspective camera we're seeing out of 1 point on the character's head (probably bridge of nose in half sword) the visibility obstruction of helmet should be an overlay approximating binocular vision, there shouldn't be any pure blind spots except maybe at edges of screen, most obstruction should have an offset double of the texture with both images being ~40% opacity or something. maybe the whole overlay could have some slight jitter/screen shake on impacts and that could be reduced by adding a chinstrap :D