@alive_gene
if you press button, in core there is update function that is looking for inputs you submitted, also for country, region & city.
If it is empty (or does not exist), it simply update value to empty.
I got 1 idea with low rate to work, but you can test (try only for city first).
- remove (or comment) location code on item-edit.php
- add somewhere on place where original location code was this one:
$_POST['city'] = 'City1';
If item updates with this city, you can define all attributes to submit post values...