I think the easiest thing might be constructing some kind of polygon-based line, that seems to be the way you would do it in a glTF model (see discussion here: https://github.com/KhronosGroup/glTF/issues/1277#issuecomment-371981447), so that way it would work wherever 3D Tiles are supported and wouldn’t require custom client shaders.
I do think having robust support for lines is one of the goals of the vector format in 3D Tiles that’s still under development.