Doesn't an accessor method generator have to make assumptions about how your object's data is stored?

Not if the generator takes care of storing the data. The generator could treat the object as if it were an inside-out object, and store attribute data in a hash - either a package variable, or a lexical hash using a source filter (a source filter doesn't have to parse the source, it can also be used to add something to the code - as what generators are basically doing). Then it doesn't really matter whether the object is an inside-out object, or a hash-based one, or something else.