I think looking at EF as you did is the best way to see availability. I also suspect that some agents may not know they need special A upgrade inventory on AA for a GUF and you may have gotten lucky. Just that definitely happened to me a few times right after GUFing AA flights became a thing. But have tried a few times in recent weeks and if there’s no A upgrade inventory I can’t apply the GUF to that segment.

And that A upgrade availability on transcons has been scarce to say the least whether 9 months out a few days out other than maybe really early or late departures…sunday morning SFO-JFK is usually available. Even Thanksgiving Day has nothing JFK-LAX. And I looked at LHR-DFW and the only availability - either next week or 9 months out - is…wait for it…on a codeshare operated by BA!