Well, after not finding any other great weekend rates in BA -- and having made a friend at the Sheraton Libertador -- I tried to find out whether I could get a deal there for my other weekend in BA. Starwood rez wasn't helpful in the US, so I emailed the hotel direct to see if they were offering the SPG "cash & points" rate for that weekend. They said they were. After a couple emails to BA and phone calls to Starwood rez, I got it all straight.
As a cat. 3 hotel, cash & points is 2800 points and US$45 cash. Seems like a decent deal.