I think that how the hackle is used determines what the trout sees. When hackle is palmered around the thorax variant dry fly style, I think it probably represents the wings. When tied on a Quigley cripple, I think it represents the legs of the emerging mayfly and the deer hair is the trapped emerging wing.

When tied parachute dry style around a post, it represents the legs and the post is the wing. Cut off the post of a parachute dry and presto, the "legs" now represents the spent imago "wings".

What we tie is suggestive and what is suggested not only depends on the material but how and where it tied. As with the cut off post of a parachute dry to convert it into a spent spinner, it also depends on whether other material is present or absent.

Whether a furnace can be used as a substitute for coachman brown depends on whether the dark center stripe will be seen as a thicker part of the fly body separate from what the brown represents. Tied to represent legs it will probably be OK but palmered, the color shift can simulate a black thick thorax with brown wings.