I have an aesthetic suggestion.  In the current implementation the fill-column char is, by default, always drawn with the background of the default face, overriding any other background such as region highlighting.  That leads to an effect that I suspect many users will find jarring.  See the attached screenshot a.png.  You might consider using the same background for the fill-column char as you do for the preceding stretch glyph and subsequent fill to end of line.  The attached b.png illustrates what that would look like.