You should be able to get them working now (in 1.0.10). First, choose the desired tonality system from the drop-down at the top of the Key Signatures and Accidentals panel. Next, create an open or atonal key signature in the score, by typing Shift+K and then typing e.g. “open” into the popover, and hit Return. Now you should find that you can happily create any accidental in the selected tonality system anywhere following that (invisible) key signature change.