Follow-up - I spent way too much time trying to make this work, and couldn't. On outbound there was no way to get seat selection map to come up, either before checking in with UA or after. I ended up asking at transit desk, and was assigned two seats together. Which were then changed by the time I got to the gate to the last row of the plane. One good thing - they deboarded from front and rear.

On return I couldn't check in until T-24 or so (maybe T-23). I think I could change my seats then. I didn't. And wouldn't have mattered . . . because by the time I got to gate our seats had been changed again, this time for the better to a window/aisle pair with the middle empty (not "first" class).

Anyway, I think the best you can do is ask for other seats at transit or check in but don't spend time on it because you may still end up somewhere else. And the EW seats are all crappy on the A319/320 - no recline, minimal legroom, etc.