If you end up resigned to riding in Y and give the pax sitting next to your wife a complimentary upgrade to F, what would be wrong with discretely letting him know the good news on the condition that you swap BP's and he pretends to be Mr. anotherbrian for the day. If you don't actually take the F seat and just proceed straight to Y with your wife when she boards, how could the FA's really know?

You're a bigger man than me, though, if you'd be willing to forego a confirmed upgrade to sit with your wife. I'd tell her tough luck, but maybe that's just me (and maybe that's also why I'm not married...).