I can't think of any place that would be particularly appropriate, unless you wante to use the GT4 race report subforum as a placeholder. A moderator would be able to move your thread to the relevant GT5 subforum if and when it gets created.
Otherwise, I'd suggest writing them up in Word and saving them. I'm assuming you want to do something like this again? Speaking as a writer and as someone who has done this kind of thing before, I'd suggest saving it in Word and building up a couple of chapters. You'll have more time to plan your story out and you'll be able to control the release of new content. Your audience will enjoy it more that way, I think. I know it has worked for me in the past.
