Indeed, I have ended up in a Club seat on a WT fare, a WT+ POUG, and an op-up. A most unlikely set of events, but I shall enjoy it.

No additional API was required at checkin (other than what might have been gleaned from the scan of my passport). BA's website is still quite confused about my trip and still says I need to supply more information, and the app even more so: it currently says I am in World Traveller seat 13K, despite refreshing it recently. I remain suspicious that the upgrade, plus op-up, confused the situation, but this sequence of events has very low probability so I doubt many others would ever be affected.