'Validity' is a question of personal justification, but asking a
question that is defined by the anwer is not likely to produce any
consequential realizations. The question achieves very little in that it
allows you to choose any set and label it, 'world.' You can say that there
is a world made of bacteria, thought, objects, values, illusions or bread
crumbs, but you haven't discovered anything in producing an answer (the
source of Wittgenstein's objection).
The question may prove useful as a thought experiment, which I
believe was Pirsig's intention. I don't think he's trying to convince you
to actually see the world as a collection of values in an exclusive or
factual sense. As you mention, it is a way of seeing the conflicts between
'Eastern' and 'Western' ontologies in another light.
>The noumenal world is supposed to be knowable through reason, isn't it?
>Sort of like Wittgenstein's "homologous form".
Supposed by whom?
The limitations acknowledged by Wittgenstein involve language
rather than perception. His assertion is that one may come to know
something of the world beyond language through transcendental insight, but
not through rational investigation. The reason that language imposes
limitations on expression is that it is inescapably rational and linear,
which makes it difficult or impossible to frame or express
non-linear/non-rational concepts.