Landscape line-trace hits underground?

I’m trying to ignore line-trace hits to the underside of a landscape so my agents don’t become trapped underground.

Is there a way to somehow make the landscape one-sided (positive normals only) so line-traces from below don’t report a hit?

In theory, I could check:

If (landscape_trace out.hit.normal.z < 0) then
   ignore the landscape_underside line trace hit

That would probably work, but it would be more efficient if I could enable some single-sided landscape option (if it exists). The alternative is to create a single-sided static mesh landscape, but I prefer to use UE4 editable landscapes.