Since you are working on the

Since you are working on the orphan mesh, you don't have any geometry information anymore. In CAE you can probably use select by Angle, but this option is not available (as far as I know) in the script environment. How complex is the surface from which you want to select the nodes. Typically you can loop over all nodes and select the ones according to some function.

In principle, I try to avoid using oprhan meshes while working with python.