Apparently, even AutoDesk (as far as I can tell, the closest thing to a descendent of the primordial Alias-Wavefront now extant) uses a 3rd party .obj import module. The developer has some opinions on some apps formatting.

http://www.guruware.at/main/ 

From a practical standpoint, for me, I'd say Animal Farm rules. Some .objs are more equal than others.  If it doesn't work in some of the most widely used 3D apps, then whether the format is, in theory, valid is academic IMO. I could probably use Base64 to include thumbnails in an .obj file for my own use, but I'd be sure to preface that data with the apparently de facto standard # comment.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken