I have been buying vintage patterns online again, and this time I think I may just have found another pattern for a Joan dress. Simplicity 5790 has a gorgeous plunging neckline, revealing an inserted vestee behind it (which could be done in the same fabric as the rest of the dress, or in contrasting colours).
This pattern really reminds me of Joan’s blue dress from Mad Men, Season 2. I reckon if you put contrasting patterned fabric in the Simplicity pattern, and enlarged the bow, then you could have a real match for Joan’s dress. You would have to fit it very closely to the body of the woman wearing it, of course, but it could be a real show-stopper of a dress.