Wow, I think AS may have finally re-opened the floodgates. I have been monitoring U availability for SEA-HNL fairly closely for many months now. And when I checked yesterday, there was still only a smattering of availability, all within the next few weeks and none after the end of this month. Today when I woke up, I saw a bunch of EF alerts I set up months ago finally fire, and sure enough, looking on both EF and the Alaska site, they're now showing tons of U availability for SEA-HNL well into next year (June, at least).
I further confirmed by calling in and modifying an upcoming SEA-HNL flight I had for mid-November. I had booked seats in G using a companion cert for my wife and myself, and we're both just lowly MVP's so we were waitlisted for F and PC. But I called in to up-fare to B, and though the rep indicated she saw the U availability, the system didn't allow her to be able to grab it for us as part of the up-fare transaction. However, within about 30 seconds of her submitting the transaction, we were both auto-upgraded into F.
Anyway, not sure about transcon or other routes, but you may want to check...