Travel on tourist visa awaiting J 1 visa stamp

[Hochschild1]

I am on a J 1 visa that has just been renewed. I have just found out that to get the stamp renewed, I need to apply at a foreign consulate or embassy, and that the process can take several weeks. I have been invited however to speak at a conference in Brazil, and won't be there long enough to apply for a visa stamp. I was hoping to return to the USA on a tourist visa (I am an EU citizen). Has anyone got experience with this? Your help would be massively appreciated!