I think it's more that we are just used to transmaps.  My character has the eyebrows on the texture, but I'm going to play around with the masking method that Gareee mentioned and may do an update to provide a browless texture option with eyebrow overlays worked into the materials.
Technology has advanced to a point where we can do more realistic options with different methods. It's just a little scary to step away from what we already know.