I flew IAD-EZE this last Tuesday. I was on an S fare, so no upgrade possibilities - but at least I snagged my favorite seat -16A. The flight was 0'ed out in all fare buckets two days before departure (as was the day before). I think upgrading on these particular flights would have been problematic. That said, I've only once not had my upgrade to C clear as a 1K.
Judging the load at the seat map is very difficult on this route, since Argentines mainly use travel agencies to book their tickets, and seats are often assigned at checkin.
UA has been more reluctant to clear the waitlist on this route because of the discontinuation of the ORD-EZE route. Of late I usually clear either 3 days before departure, or more rarely at the gate.
John