I still use DP 3.1 and just have read the 3.5 version history. The chapter about the line style editor is tickling me. Is it possible to cap a line with another editable line (which can be capped on its own), so I can create a barré indication for guitar? If not, I wonder if this is an easy feature to add… 
No, you can’t have both a horizontal and vertical line as part of the same item. We do plan to add guitar barre indicators as a dedicated feature at some point in the future, but I can’t say when it will be.
I used the Line Editor - Vertical category - Solid vertical line with right-pointing hook.
Then I changed the Start cap to ‘right-pointing axis-aligned’ and the End cap to 'None".
This now shows up as the barre line and I can alter its bottom and top in the Properties panel.
The C barré indications I created in the guitar playing techniques.
Does that help?