Polylines coud be elevated but not (yet) from a user interface.
In polyline panel, after the view in 3D check box could be added elevation and arrow orientation (horizontal or vertical).
That's only if Emmanuel make this changes in the main program.
Set elevation is simple but arrow orientation it's not that simple.
How I've test it:
-install Generate roof v.1.0 alpha (where I've use elevated polylines)
-generate a roof with inner polygons.
-save file.
-I've remove all plugins to see what happen.
-Reopen file and polylines keep their elevation.
Indeed this option could be added to Advanced Editing -> Points Editor but also it's not that simple.
A workaround is to edit manually Home.xml on every file and add elevation tag with value but is very difficult.

When I've renovate my apartment I've solve it with elevated cylinders and push them in walls.
In 2D plan I could see it and in 3D I make walls transparently.
In that way I could show to constructor what's in my mind and also I've have approximately the length and quantity.
Example for heating