This was previously semi-supported. There were two problems, however:
1. My fill-column value of 80 got out-of-sync with my white-line-column
value of 100.
Solution: globally define wpc/fill-column, set that to 80, and
consume the new const in my config.
2. Nothing was activating whitespace mode.
Solution: add whitespace-mode to prog-mode-hook.
This SO answer helped tremendously:
https://stackoverflow.com/questions/18855510/have-emacs-highlight-characters-over-80
6393e47c77