A few of the cruise lines are going between baires and Rio this spring and summer, although it looks like most are doing it in reverse, starting in Rio.
Holland America's Veendam is making the run in early December. Although the cruise is officially between Santiago and Rio, you might call them to see if you could book just the last days. Same for the Star Princess in late December, on a cruise that originates in LA (!) Probably others as well.