Hi! I want to add 3D to a 2D polyline wherever it intersects with a 3D contour line

Have you tried relating the lines and see if the Z value can be transferred to the 2D feature?

A surface draper should work here, might need some additional work to handle the z values at the start and end points if they don't intersect a line

Interesting. Can the SpatialRelator use the z-value from the 3D line and transfer that as a z-coordinate for a specific (=intersecting) vertex, and not just transferred as an attribute (or a z-value) for the whole 2D line object?)

Just throwing an idea out there: the SurfaceDraper

This is a lot easier indeed!