I'm a Final Fantasy demi-god and I say NO. There's nothing in the English syllabus that sets out guidelines for studying games so you could easily miss elements that would be considered critical. Besides, while Yuna's pilgrimage is a journey, the plot is very complex and involved, with plenty of twists. Explaining some of the key concepts - such as Fayth, Aeons, the Unsent and Zanarkand - would take up too much space in an essay and while they're crucial plot elements and would probably be necessary in such an essay, they don't really add to the concept of the journey. You realy have to play the game to be able to understand it fully - I could write up a detailed synopsis of everything relevant that happens and an outsider would be completely lost probably by the time they got to Tidus' conversation with Auron at the docks in Luca. Which teacher is going to play a video game just to understand a student's essay?
Besides (and most importantly) it would look like you're shirking off study simply to play video games. FFX is a long game - 50 hours for me to get to the final battles with Sin - so unless you've played it through several times and/or have access to the game script (which isn't that difficult) you're not going to be able to do much. Plus, there's a lot of the actual game that isn't really part of Yuna's pilgrimage - Operation Mi'ihen springs to mind, plus the tournament in Luca and the trip to the Farplane in Guadosalam - and it would be easy to get caught up in these parts when you don't need to.
So I'll say it again: don't use a video game as an ORT. It jsut looks like you're not taking Year 12 seriously and would rather play games than study. That's how I'd see it if I were a teacher.