Hobbitfeet
I think you have pretty much answered your own questions there.
I doubt you will find any such documentation outlining visual lean on specific parts of pork. As you state, the variations are just too vast.. Not only are there variations between breeds, but variations within breeds, dependant on a lot of factors. Feed rate, excerise, age/size etc..
Failing that...how is the VL of any cut established?
Visually?..
Regards, Paul Kribs