You're right, you can actually use the curves trick here, where you first convert the mesh to a curve, and then back to a mesh.
However, that doesn't answer the question, so here's the answer to the original question:
To align the normals of a face uniformly in this case, you would need to know in which direction the normals should point.
To do this, you would need to check the dot product of the position and the normals of the faces. If this value is greater than $0$, then the face points outward. However, this solution only works for a mesh like this one, where the faces are reasonably circular on a line.
If this technique doesn't work well because of the mesh, it can still be extended by using a common center point as a guide:
By the way, I've created a separate vertex group here that includes only the points along the edge. This way you save yourself some more nodes.