Thanks, @chrisb and @jmaustpc (and others!). It's nice to be back, even if I'm only slowly getting up to speed.
I'm not sure how the fancy high end commercial CADs handle this, but IIRC, AutoCAD requires you to edit isometric dimension after creation to adjust the angle with the DIMEDIT command. We could perhaps add an oblique angle property that would control the slant of the extension lines.