My guess, and it is only a guess. Oregon has fairly mild winters if you are on the west side of the Cascades. Sap would continue to flow in the understock, and nourish the scion all winter. Based on comments by various authors, if I read it right, pines will keep sap moving as low as 28 F. They won't be growing at all, but sap can flow (slowly) at these temperatures. So even though Oregon will dip below freezing especially at night, I don't think the west of the cascades area gets very cold at all.
I do know commercial landscape nurseries will harvest scionwood in January or Feb. Store is in cold storage (below 38 For 4 C) holding it until about bud break time for the understock they want to graft the scions too.
You are in SC, it may be much easier for you to provide a wintering site where the temperatures would stay above 23 F. If you can, there is no reason not to try fall grafting.
I took a grafting class at a landscape nursery, and one of their secondary grafting seasons is middle of August to first week or two of September. This was a Wisconsin nursery. The success rate was good, not as good as the early spring, but good enough for their commercial purposes. One reason they do grafting in the late summer, is because the spring work load does not give them enough time to everything that they need to do. Because of climate differences, late summer in Wisconsin might be the same metabolic timing as autumn grafting in Oregon.
Someone who knows more about pines should verify that either I got this more or less right, or correct me on this, because my grafting success rate right now is at about 18%. Pretty low, though for the majority of my failures I can identify what went wrong. For those, practice will correct the issue. But there are a few that failed that I thought I had done right.