This looks nice, I had been thinking about something similar, as mine is attached onto the towbar, and when I reverse a bit too far, the filler grounds, and get damaged on kerbs etc. However, my local filling station has added a stiff plastic flap around the filler, presumably to control the gas that escapes when you uncouple, about 7" dia, which would foul both the top of the number plate gap and the number plate its self. Hingeing the plate at the top would be a bit easier, but it would restrict your vision when coupling up.
There doesn't seem a better place to fit the filler. All I can think of is to fit another petrol filler on say the other side, but have a gas point in there. This would need a chunk of cutting, welding and painting to look good, with the advantage if you wired up the solenoid, nobody could get access to the cap unless they forced it. However it might still foul a flap like I have described. If I remember, I take a photo next time I fill up.
Ken