Generate Landscape Actor beneath the OSM data by Hemofektik · Pull Request #16 · ue4plugins/StreetMap · GitHub (closed PR) is the only code that is still publicly available at the moment. Some of the work has been removed by the author unfortunately. It will hopefully be up again, soon.
Generating landscape splines from OSM data is possible, I grabbed his fork while it was available. I have it working on my drive. It can be released soon hopefully. In its current state the editor performance is really really bad with so many landscape splines having the green helper mesh / gizmo (don’t know how to call it) rendered, which makes them unusable IMHO. If they are disabled its okay, but editing them is not nice this way. I’ll keep you posted, when its in a usable state.