I often use Harmony to create illustrations and background drawings.
When doing this I don’t separate Line Art from Colour Art (BTW I do not systematically separate them in regular rigs neither) in order to speed up my workflow. All my art work is drawn on the same art layer.
However, it happens a lot that I would like to select only the lines (e.g. in order to edit their colour). That’s why I think that it would be great if there was a mode allowing the user to select only the lines/fills independantly from the concept of art layer levels (Line Art, Colour Art, …)
Another approach could be to have the possibility to unselect lines or fills (=strokes?).
And, of course,it would be great if this function could be applied to several drawings (nodes) at the same time.