You're blowing this out of proportion
My school also allows a bilingual dictionary for proficiency exams but like StrangeLight said, its best to contact the DGS or the department in charge of administering the exam. At my school the exam is a scholarly text of one or two pages that has to be translated. I've seen the exam for Spanish and if you know Portuguese well enough, you should be able to pass. However, should passing be your goal? Since you're a Latin Americanist, knowing Spanish well will help immensely in the future. If your interests shift at all during your program there is a good chance that you're going to be using Spanish. It will also make you a stronger candidate for jobs in the future. (And normally PhD programs in LA history require Portuguese, Spanish, and French, no?)
Also, I have noticed that the culture of some Latin American departments are Hispanocentric (I think I am inventing this term but you get the gist). That is, Portuguese speakers are usually outnumbered and often end up communicating in Spanish, guests are often invited to give lectures in Spanish and rarely are lectures held solely in Portuguese (yet I'm sure this depends on the department). Knowing how to communicate in basic Spanish will allow you to take advantage of more opportunities in the future.
I bet if you took an accelerated Spanish course over the summer and put some Shakira and Calle 13 on your ipod you would pick it up in no time.