So the CW seems to be to let the RCC desk agents know one is waitlisted (so they can "keep an eye on it") or to make sure one is at the gate area 15 mins prior to boarding. I'll probably do both in future.
Thanks for all responses.

So yesterday, I made it to the gate with Mrs. Ants - we were last to board because I'd apprently spent too much time posting to FT in the RCC . The GA stopped me as soon as my BP was scanned, and told me my u/g had cleared, and handed me a new BP. Only one of us cleared .
His explaination was that my wife was much lower on the waitlist due to her status - 3P. He also told me that my E+ seat had already been given away / reassigned. (To standby pax? Flight departed full.)
So that would have put Mrs. Ants in the E+ exit row, and me in F. Not acceptable. So the only option I saw was to give away my F seat to the 2P who received my original exit row seat and join Mrs. Ants in E+, which is what we did.
Lucky guy, eh!? I'll have to consider my lost five E500s as compliments of the season to the stranger. (Who apparently was so shocked to have a F seat given to him that he didn't think to offer any thanks. I suppose he was doing me a favor )

Lesson learned. In future I will certainly be around the gate area before boarding when waitlisted for an u/g. At least that would have given me the option of declining the upgrade if only one of two clears.

I am curious, though... Shouldn't Mrs. Ants have been right up there with me on the waitlist, regardless of her status? We were on the same itinerary. I know it's possible that only one F seat opened up, but it's the GA's comment about her status that has me confused.

And if anyone has any suggestions about how I could have better managed the unfortunate situation in which I found myself, I'd be very interested.
(And if that suggestion includes comments concerning how I might be able to recover my E500s, even better )

Thanks again,

Ants