The DPLineStyler extension was just released. It lets you make dashed lines, lines with symbols, or a combination of the two. Settings are customizable, and you may have as many styles as you want.
The styled lines are implemented as groups of edges and symbols, so they are independent of layers, and you can go into the group and tweak things like dash and symbol placement if you need to.
Since the lines are made of actual model geometry, they will maintain their appearance as you zoom in and out, or export to layout, PDF, and CAD.
Runs on SketchUp (Pro & Make) 2016 and later, and on Mac and Windows. See it here!
How does it deal with a line that changes length? Does the style remain for the lengthened/shortened line?
You erase the old style group and make a new one. 
Every style group is named with the style name and the scale that was used to make it.
Just did a minor update to DPLInesStyler, plus added a promo video on YouTube:
