With great difficulty and a hell of a lot of luck!
Here's the insides of a standard cap which might help you visualise what you're up against -
Click on the images to zoom in if needed.
I have no idea what the locking version looks like. When it's locked, does the chrome part rotate or does the whole thing sit solid?
If it turns, then I'd think the locking mechanism will simply disengage that plastic nub (seen in the 2nd pic), allowing the cap top to rotate without the bottom part turning. If that's the case then you could try making a pointy hook that bends under the lip of the chrome cap that grabs onto the plastic bit underneath and try turning the whole thing with that.
It's a bit hard to explain what I mean.
Failing that you might have to start drilling in various places. As long as you stay within the inside diameter of the filler neck you should be ok. Maybe one 1/4 inch hole about 1/2 to 3/4 inch from the centre about 1 inch deep, then slide a screwdriver in there and try turning the cap.
The cap will be destroyed, but at least you can fill up again.
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