Do Custom normals work in Animation Nodes? Generally I want to direct polygons along custom split normals, or at least get them work in AN. Can it be done?
You can get access to loops normals using the following expression:

[loop.normal for loop in [obj.data.calc_normals_split(), obj][1].data.loops]


Which calculates the split normals data and returns the normals. To set vertex normals to a mesh, you can use the setVertexNormals method, which take a single vector list representing the normals.