I can't think of a reason why they would have -- though the ticket was "purchased" using MP miles and issued by UA, the ticket would have been under the control of Asiana, and as the operating carrier OZ would have been in the best position to re-accommodate you. An exception would be if this was a route that both UA and OZ operated, in which case UA *may* have been able to be a little more generous with their own seats. But in this case since it's not a route UA operates there would have been no reason, other than another point of possible confusion, to pull UA into the conversation. Plus, airport sales agents aren't specifically trained on MileagePlus and the nuances of awared redemption, much less interline award redemption, so the chances of them being of any value for an OZ operated flight are slim.

Although it's unfortunate you missed the flight no part of the situation was UA's fault/responsibility [if you used a credit card {without travel insurance} to buy the ticket instead of miles, would you expect compensation from the CC company? That's basically UA's involvement here] , and it seems that Asiana did everything they were supposed to do [and maybe it's my bias towards flying last-minute on high fares, but US$400 sounds very reasonable, even cheap, for a walkup/last minute business seat, so maybe Asiana did do you a favor behind the scenes] -- fault and potential remuneration, IMO, lies with the taxi