it just isnt, because rap music is pretty broad by definition, u'd just have to make a definite idea of what it is...what about getting like eminem's early ep's and stuff, and other people's.
my number one reason it cant be done is because as soon as someone would think they have every rap album out there, they wouldnt have everyone. how could anyone make a list of every single thing released? think about worlwide releases.
getting every rap album released in the bay area would be hard enough, every single one released ever? no way.