r134a Is it programmatically possible/feasible to 'intercept' a request to that api and have it return another value or null?
Yes. "Simple matter of programming".
Except that once the code is written it needs to be ported and tested for each new upstream release. Historically GrapheneOS, unlike other Android variants, has kept up with monthly AOSP releases from Google. If you're merging against upstream only every six months, then naively you're doing something like 1/6 of the maintenance work of keeping up monthly. For GrapheneOS every added feature costs maintenance, so it's necessary to prioritize. I think at the moment there are five or six open issues about spoofing or blocking app access to various things, some more threatening than country code.. Also at the moment Google is making things harder in unpredictable ways.
So this is straightforward and easy, except not actually easy. A year from now perhaps things will have settled down and perhaps the team will have a full-time spoofing/blocking developer. Who knows?