From the first year that the reciprocity fee was in place, it was certainly collected on arrival from Brazil, from a friend of mine who holds a US Passport and Brazilian residency (together with whatever the appropriate card from Brazil is).
He was a little surprised, since he had the impression that his document from Brazil would have been sufficient by itself to travel between Mercosur countries. The problem might have been that his US passport was already used to leave Brazil, or his Brazilian document might have shown a birthplace that made them decide the fee applied. Either way, probably no way around it, but at least it's a once in ten years problem.