Typically when I have been in the situation you are describing, the F seats don't ever show up in U inventory, but, I get UG'd at or after online check in, before getting to the airport. I have gotten UG'd at boarding also, so never give up hope.

If you have the smartphone app you can watch the UG waitlist once you've checked in online or through the app.

That said, transcon comp UG's are few and far between. If you're an MVPG and you bought your ticket a long time ago, maybe you've got a shot, but AS sells a lot of paid F, and any 75K will get ug'd ahead of you. FLL might be an easier route to land a comp UG than my usual transcons - DCA and EWR, the hardest in the system per reports here on FT.