SMS is right. It can't be done. If you want to upgrade using miles (or a PMU) you will need to buy a K fare or higher, which will likely cost half again as much to twice as much on a route like this. Plus there would have to be G inventory available now (not V inventory) to upgrade using miles or a PMU.

Since you already mentioned that cost is a factor, it sounds like this may not be an option for you. So my advice is to book yourself into the best exit row seat you can find and hope for the best. Your upgrade chances as a Plat on a T fare vary widely on a route like this depending on the day of your flight. If it is a Monday or Friday flight, you'll be lucky to score an upgrade. Other weekdays might be better and you are almost assured of an upgrade on the weekend.