I've had this problem for a long time. It looks like it's common (the manual even explains how to manually open the door).

Today I finally got tired of emptying the cargospace and pulling the paneling out every time I refuel. I bought a set of Torx bits at Canadian Tire for $5 and went at it. 600 torx screws later and a few broken plastic fasteners I had the whole right side of my cargo space down to the metal. Lo and behold I COULDN'T GET TO THE FUEL DOOR! There was a single screw holding in what looked like the lock, but I was too chicken to unscrew it given that it might have been impossible to get back in in the right place.

So, my question is, if I spray some WD-40 in the hole locking plunge hole, will I break anything?

On a side note, the lock seemed to work when the fuel door is open, but jammed when closed. So I figured the metal loop on the door must be rubbing the lock (probably due to some gas pump jockey reefing on it). I bent the metal loop out a bit to recenter the hole on the lock to prevent rubbing. This seems to have fixed the problem for the three or so tries I gave it today. We'll see how it goes when I get to the pump and Murphy's Law is applied fully.