@Archie22is, you haven't mentioned in what setup you want the overwrite made.
Anyway, I can't see how an override of the geolocation locked address could be made based on conditions from the case you describe.
That's ok, but you seems to have locked the costumer's billing address to the geolocation, that's your main problem.
You also seem to have disabled the option to ship to another address, that's a good policy. This you could change to solve this case, but then you opens for fraud if the account is compromized, not so good.
I don't think you can make a conditional rule that apply to this case, but I may be wrong.
Maybe you could try a "Multiple Shipping" extension to care for proxy customers or gift shopping, but I'm not sure that would work if the shipping is locked to the billing address.
The only "overwrite" solution I can see is to inform the customer, if he isn't a US citizen, to contact the shop to manually set his account's address. (E.g. that message could show in a custum field before the country address details are entered.)
But I strongly suggest you to remove the geolocation lock on the billing address. It doesn't improve or protect anything, and IMHO it's only useful if the shop only ships to its own country.
I believe the geolocation should rather be used to suggest an address in order to make the registration easier at checkout. Keep disabling another shipping address though.