jonny walker is a US keeper who plays in Chile, i don't know of any current field players, but lots of people on this board know a lot more about YAs. i know in the past several US players have played in Mexico and Central America but I don't really know about S. America. hugo perez i believe played somewhere some years ago, but it might have been in el salvador or something.
From a recent Soccer America article on Yanks playing in South America: "A lack of demand and economic instability make Latin America an unlikely destination for U.S. products." The article goes on to list Americans past & present playing South of the Border: http://sportsillustrated.cnn.com/soccer/us/news/2003/01/14/us_abroad_sa/
you know it's quite curious how topics like this start up and there's an immediate answer following...it almost seems like the writers of these articles start these topics and shamelessly plug their work!