Most of the availability to GRU seems to be on BA234/235, I don't think there are any available spaces with through connections. (At least for the few dates I checked.)

Interestingly enough the ba site indeed doesn't allow you to specify a stopover on the way to EZE...

BA does sell the GRU-EZE legs, and I even had trouble getting a EZE-LHR flight on my DONE4 because the EZE-GRU leg was full, where there was plenty of availability GRU-LHR on the same flight...