Doing it with a Playing Technique is a pretty easy workaround. Create a new PT, for Type select Glyph, hit the Edit Composite pencil, delete whatever text is in the editor, for Range select Barlines (3rd option from top) and add a double bar. Hit Ok.
Back on the main PT page select Below for default placement otherwise Dorico will try to avoid collisions with the chord symbols on beat 1. Add the double bar, switch to Engrave and move it into position.
