Update: Of course, as usual, I did this without checking to see if there was already a PopClip extension for CriticMarkup. I haven’t tried the one that comes with the download, so I can’t say if mine adds anything worthwhile or not. If you have input, let me know.
This is a companion discussion topic for the original entry at https://brettterpstra.com/2013/09/08/editor-and-criticmarkup-extensions-for-popclip