Great news ‑ core code and RNA API are in trunk (commits r60005, r60014 and r60015)! Many thanks to Campbell, who helped a lot optimizing and cleaning up the core code, and to Brecht, who reviewed the API code. 😀
Note that for now, there is no preview in 3D View, and no storing of those split normals in .blend file, as this is only temp data currently.
Very soon (probably tomorrow), I will commit the OBJ update. FBX will have to wait a bit more, as it first needs some deeper changes! Anyway, hard work is done, py scripting is easy… 😛