I am also waiting to hear from Brazil ETA and I thought I would be in the same predicament but fortunately the other fellowship to which I applied has also been delayed in notifying applicants. When I discussed the timing issue with my mentor a few months back, she advised me to accept the other fellowship. Worst case scenario, you'll get Fulbright and break your contract. It sucks to have to do that, but people break contracts all the time. I'm sure the other fellowship has alternates; you won't be screwing them over or stripping another applicant of the opportunity. If you turn it down and then don't get Fulbright (God forbid! I'm crossing my fingers that you do get it!), you'll be without any fellowships! I thought this sounded a little selfish and dishonest but you have to look out for yourself. I've also realized that this is fairly common. I just learned of a girl at my school who signed a contract with TFA and left during the middle of summer training when she was accepted as a Fulbright alternate. TFA understood--Fulbright is really prestigious and a wonderful, once in a lifetime opportunity--and they renewed her contract when she returned from Turkey the following year.
I agree with tieloran, though; I would wait until the last possible moment on Monday before letting them know. And I would try calling the Brazil program manager. I emailed her last year week and haven't heard back but maybe she'd be more responsive via phone.