Jessie
From my understanding, the answer is No.
Your Permanent Residency card will have your country of origin on it and say "extranjero" across the top. It's my understanding that you need to be an actual citizen of a MERCOSUR country to work in Brazil and it will be obvious from the card you will have that you are not (the Brazilian authority uses that card as a point of reference)
As Iamarque said, there are many Uruguayans that live in Argentina and work, however it's also my understanding that they probably went through the process to obtain an "DNI" with the Argentine government to legally work here.... The Mercosur card just allowed them to come. You would have to do that through the Brazilian government and most likely would be denied if you don't have an Argentine spouse working there with you (which grants you temp. residency) or have some other type of visa granting temp. residency specifically for Brazil.
I might have changed or there could be another way, but I had the same question for myself a year or so back and I found different resources that explained this.