I've been reading this thread for a while and thought I'd chip in my 2c after owning a Mondeo with capless refuelling.
Nobody is going to carry a PVC pipe around with them in the hope they might find a Fiesta with capless refuelling to then try to siphon fuel or inject something into your fuel tank. It's so unlikely as to be not really worth your time investigating surely! The Capless setup has a built in lock that needs all the tabs pressed at the same time to open the flap from memory.
Besides that, if I was a criminal stealing fuel, I wouldn't be targeting 30L tanks in Fiestas, I'd be looking for something bigger to get more fuel to make it worth my while. Just in my honest opinion the combination of the built in lock on the capless mechanism, the size of the tank and the fact that there aren't nearly as many Fiestas around as other cars, it'd be one of the last cars in the world anyone would ever target.
If they steal the funnel out of your boot, then they've already smashed a window/boot/busted a door lock. Not questioning your right to buy a locking cap, but I'm just not so sure it's a massive concern.
If in doubt, check
www.european-parts.net to see if they offer something, otherwise do a Google search for one of the Ford UK Dealers and drop them an email to see if they might know someone over there who does one if the US ones don't fit.
As a thought, if there a locking pin built into the fuel-door arm then perhaps a Euro-spec Titanium might have this in-built like a Falcon has here? Get one of the UK guys to check their catalogue?
Hope it helps.