Thanks @will.schroeder! This is really nice. The only thing I would propose to change is that the BoundaryLabels does not sort the labels. This makes it impossible to directly extract the triangles with correct orientation. Instead if you simply comment out this line the orientation of triangles/quads is preserved and no post-processing to generate consistent normals is necessary.
I created an issue here: https://gitlab.kitware.com/vtk/vtk/-/issues/19156