On movie posters you often get little boxes with consumer advice in them like "15 for Strong language and violence" or "12A for mild sexual references" You also get boxes on the back of DVD cases with a description of how much sex/bad language/violence etc the film contains. The BBFC also have the PBBFC site with advice for parents.^ I was actually thinking about using "U" for my "N".
Yeah, it would be along the lines of the BBFC but having additional descriptors after the age rating would be key.