The documentation suggests that this data exists somewhere internally but there's no way to get it.
http://help.autodesk.com/view/MAYAUL/2018/ENU/?guid=__files_Polygon_API_How_polygons_are_handled_int...
The structure that holds UVs in Maya consists of two arrays:
A UV index array that uses the exact same indices of the face array (visualized as a face vertex array).
A UV array that holds a list of UV points indexed by UVIds.
The currently available api methods (that I'm aware of) queries faceVtx individually and are simply too slow on a large mesh. Especially when doing this via Swig/ API2
http://help.autodesk.com/cloudhelp/2018/ENU/Maya-SDK/cpp_ref/class_m_fn_mesh.html#a1d2b8c1536e8f4ec3...
http://help.autodesk.com/cloudhelp/2018/ENU/Maya-SDK/cpp_ref/class_m_it_mesh_polygon.html#af6930b5fb...
http://help.autodesk.com/cloudhelp/2018/ENU/Maya-SDK/cpp_ref/class_m_it_mesh_face_vertex.html#aa8d54...
