Reade, I have been studying Trilliums for some years now. I have never seen this before. That area is under the closet, on the inside. I would be inclined to sand the inside, so you get good adhesion of a couple of new layers of fibreglass, on the inside. Not sure how you would get access to it, other then cutting out the bottom of the closet, or putting in a door on the lower section. This could provide some access, but you would still have to reach over the wheel well.
There may already be a door there, but it would have a lining that you have to remove. There are from one, up to three doors on the closet of a 1300, depending on year, and options. The older Trilliums wasted lots of space at the top of the closet. Later ones all had small door at the top. The lower door, being in-front of the wheel well, is so shallow that other then a spice rack, not much would fit there.