I'm not sure I have a clear idea on what purpose the rest matrix/bindpose is supposed to serve in skeleton animation. If it is supposed to transform a bone into the rest pose and the mesh itself is always exported in the rest pose then shouldn't the rest matrix always be the identity matrix? But then why is it not the identity matrix in Blender?
I also find that if I transform my mesh (exported in the rest pose) by the rest matrix for each bone then it is very incorrectly deformed (it was after all already in the rest pose).