Accordion Notation - underline counterbass

In accordion left hand notation, at least in Germany, it is common to underline bass notes that shall be grabbed on the counterbass row. When I write leadsheets for accordion I use slash chords to specify bass tones if necessary and would like to underline some of the bass tones, like this

When I try to underline using the inspector, I can select only the complete chord symbol instead of only the bass tone

The resulting underlined chord symbol is displayed rather ugly

Did I miss something? Any help is appreciated.
Here's my example as MSCZ:
BriseN Snippet.mscz