Well… Three years later ^^… I’m on the same problem today on my side !
Sorry that I can’t solve your problem, but as I’m here and the topic is still unresolved, I bump it.
I’m pretty sure it can be done easily to trace a vector that adapt its trace accordingly with the terrain elevation.
But unfortunately i’m not a big expert on coding.
So if someone’s passing by on the topic, I would be very grateful for any little tricks.
I post my setup equally and a beautifull sketch to illustrate the problem 
(I think it’s exactly the same problem as the author’s)
I post