I.e. promoting polylines to 3D objects...

Not exactly what you want but you can use a box to simulate a 3D object polyline:
Draw a long thin box and make all sides invisible except the top. Export and import. This leaves you with a "line" that has a 0-height because it's only a single face (the top) of the box. You can handle that object like any other object, i.e. move, rotate, resize, elevate, color/texture, etc. (except the height of course).
I often use boxes instead of polylines because I can handle them like any other object, specifically grouping, give a name, and make them invisible. In some cases the 0-height of the single face object is an advantage, in other cases I use plain resized boxes.