I took a closer look at one of our 757's last night. That access cover falls in the middle of a seam on the canoe fairing. There is an internal doubler and the fasteners in that doubler follow the perimeter of the cover. My guess would be it's a trade off for strength. The small round area in the center is the only part that really disrupts the structure. Plus you save a few screws and nutplates.
The panel is for a the bolt that holds the aft end of the canoe fairing. I can't recall if it has lube point on the end or just fittings inside. I think it is the strange shape due to the internal structure. With the pivot motion I don't think there is room for more fasteners. BTW installing that fairing is a real pain. Hardest one on the wing.