How to add structural trend or non-ctct structural data to a mesh without closing it?

I'm trying to create a mesh to use as a vein reference surface (it's a near-vertical vein running NW-SE for roughly 700m). I have a polyline based on some mapping, and some structural measurements. When i create the mesh with the polyline, it creates a mesh which looks pretty good. I then try to add a structural trend using my mapped data and Leapfrog now takes my nice planar mesh and tries to close it, even though the structural data follows that same roughly NW-SE trend. I've also tried adding the structural data directly, but of course LF wants to consider that as contact points, which they are not.

Any help is much appreciated!