I'm trying to use more whole grains - can I substitue ww pastry flour for some or all of the crust? Or other flours, for that matter?

I would say you can probably go half and half in the crust. I wouldn't substitute in the crumb topping because I think the whole wheat flavor may be too pronounced and thus take away from the traditional crumble taste. But you can definitely experiment. I'll check back to see whether anyone has a more tried and true method.