Well, the short answer is no one knows.
In 2001 and 2003, students were asked to write a creative writing piece based on their prescribed texts, so it definitely is a possibility.
To be sure, be prepared for both situations. I'm with you, I hated writing about my prescribed text as a creative piece - it just felt so lame to me.
You probably only would need to prepare one piece from one prescribed text, however, so it's not that bad.
Good luck though