MAY 2016: See Y.A.V.N.E. by Umdee. http://blenderartists.org/forum/show...8Y-A-V-N-E-%29
MARCH 2013: Please see felixSchl's thread as he is implementing something like this in C.
Easy download: https://dl.dropbox.com/u/398143/blen...vertexnorms.pyOriginally Posted by Updates
I would like to create more tools for editing vertex normals than this, but here's the first simple operator intended to serve as an example for the usefulness of such a feature. Though modifying blender's source is a bit beyond me at the moment, eventually storing the custom vertex normal offset in bmesh/mesh data and having it automatically affect rendering and export would be very nice, not to mention facilitate more interesting vertex normal editing tools.
Feel free to offer feedback and suggestions. The video below explains the operator and depicts three example use cases. This may also be useful for normal map baking; I have not yet tried it for that.