You're getting some bad advice here. Brazilian regulations were changed about a year ago. Brazilians can now enter and leave on foreign passports. You only need to provide your RG (National identity Card, for the information of those who aren't Brazilian) together with your foreign passport. No hassle at all, and they don't need to show an entry stamp since Brazilian documents are often not stamped anyway.

While the above information was formally announced by PF actual practice can vary, especially at GRU. I strongly advise seeking clarification with Policia Federal at you planned airport of departure. They are usually very helpful. The also can arrange emergency Brazilian Passport replacement, which took only one day recently for my brother-in-law.