Well, HUCA resulted in a premium cabin seat (no explanation of why it wasn't automatically available, and the Web still wants $75 to change to another seat) but no upgrade.
ExpertFlyer and the Web now agree that U is gone, though---with no upgrades processed! Looks like AS pulled U availability (from 5 to 0) at the window without actually giving any upgrades.
Ah well; hopefully the gate upgrade process works properly...
FWIW, does seem like there is a lag between when AS internally updates U inventory and when its own website registers that.
Will see if the return processes more smoothly.