Dropping by to mention, that the whole change, requested for more than a year in this thread is defined as follows:
Spline render mesh needs to be constructed in a such way, to contain vertices and edges from both splinemesh and landscape LOD0 beneath it.
A project spline to landscape button should be added.
When the button is pressed, for every spline render mesh vertex, determine the corresponding landscape triangle, get Z positions of its 3 vertices, interpolate, depending on where render mesh vertex is located in relation to landscape triangle, considering XY only, and move spline render mesh vertex to the resulting height plus constant bias.
Translucent sort priority is already there.
Dealing with with pushing landscape splines upwards in the distance, to account for for landscape LOD morph should be done by users in decal material.
That is pretty much it. Not that much of a work for one of the top requested features.