directionless bones boldly states: "there is no relationship at all between being a materialist/idealist about ontology and holding any particular theory of sociohistorical causation."
I'm not a serious philosophologer, but the idealist philosophy I've read is either complete bullshit (theism), dualist (Plato), uninteresting solipsism (Berkeley) or indistinguishable from materialism.
If directionless bones' statement is to be true true, then it would seem there's no difference at all between materialism and idealism, because of course any ontology is the ultimate explanation for all phenomena and their (causal) relations. And perhaps this is the case: as Pirsig notes, all monistic ontology (metaphysics) is ultimately about the same thing — the "one" — and merely differ on the attributes. And if there's no effect one's theories of socio-historical causation, then there aren't any differences in the attributes. Perhaps directionless bones does not disagree with the content of my ideas, he simply prefers a different label.
But of course there really is a difference between a lot of idealists' and materialists' theories of socio-historical causation.
Idealists — dualistic idealists — typically look for objective* normative universal principles, principles somehow outside of and acting upon ordinary reality, principles that either do in fact guide human history, or inform people how they ought to behave.
*independently existent in the ontological sense, and independently determinable in the epistemic sense
For a scientist, the idea of an objective normative principle is incoherent: there's no epistemic method for determining objective normative principles, since an principle can be non-trivially normative only if exceptions can actually be observed, but observing an exception invalidates an ordinary physical universal. The law of gravitation would be proven false if (absent other factors) we ever observed a rock not falling when we dropped it; a normative prohibition against murder, however, makes sense only because people can and do in fact murder each other. (We don't have strong normative principles or laws prohibiting psychic mind control precisely because it's physically impossible to control someone's mind psychically.)
So a scientist has to see normative principles not as guiding human behavior and history, but as emerging from human behavior and history; a result, not a cause. Normative principles are properties of our material, physical minds.
So if directionless bones' statement is true on* some specific interpretation of "idealism" and "materialism", then the concepts themselves do not differ, only the labels. If the concepts differ in any meaningful way, then they ought to have a profound effect on one's theories of socio-historical causation.
*with apologies to my commenters