I never did finish pursuing this, but with the help of a local Sons of the American Revolution member—who is also, like me, a HODAR (Husband of a Daughter of the American Revolution)—I seem to be about to make it into the SAR on the strength of one William McGehee who signed a Revolutionary document of some importance in 1779 and was, apparently, my great-great-great-great-great grandfather after all. I’d been unsure about this because a genealogical researcher had told me years ago he’d been unable to document that link, but my SAR friend says it’s good. Apparently another descendant of one of my documented ancestors succeeded in satisfying SAR’s requirements, which smooths the way for the rest of us.
I know that William also owned property next to the Shadwell, Virginia plantation where Thomas Jefferson had been born, buying it while the future architect of American independence was a teen. There is also a William McGehee in Jefferson’s records as having roofed Monticello, though that is probably a namesake son of this William. I would be descended from one of the younger William’s brothers.
It will be interesting to see all of the information my friend has developed in working on this for me. It should make the stalled SUVCW thing too easy to keep putting off.