Spline projection can of course create a flat projection based on your spline, but that would only work if your Spline Mesh was based on completely flat geometry (e.g. road, river). If your geo has ANY z height at all, this of course doesn’t work.
The core issue is, afaik, there’s no way currently to get the actual mesh (and its points) you create with Spawn Spine Mesh.
What Epic would need to create is the equivalent of Surface Sampler but for Spline Meshes (or support Spline Meshes in Surface Sampler).
If I’m wrong and there IS a way to get the actual procedural geometry created in Spawn Spline Mesh, definitely lmk. But as far as I can determine, there simply ISN’T a way to do this! Which is sad, b/c it limits how useful procedural geometry can actually be.