"Most people emigrating to South America in the late 19th century & early 20th century did so through the Baron de Hirsch JCA organization ..." is right, but in my opinion most people didn't arrived to the agricultural colonies, or left in a short time and moved to the big cities to begin a new life. So it is probably that JCA has no records about those people.
On the other hand, not all the records of JCA' applications have been added yet to Jewishgen.
Helen - you can contact the Jewish Community of Rosario here: https://es-la.facebook.com/kehilarosario. May be thay have their own records about marriages, births and burials.
I have a list of the JCA personal files of the colonists, you can contact me also.
YEUDKIN/YUDKIN, SCHEIMAN from Gomel in Belarus
DUGATKIN, SHMUKLER, KOHON, BERKUN, OSCHEVEROFF/OSHEVEROV, SILBERT from Kherson Guvernia in Ukraine