I think I’m almost there using a custom filter function in functions.php
function my_post_object_query( $args )
{
$args['author'] = '2';
return $args;
}
add_filter('acf/fields/post_object/query/name=my_post_object_field_name', 'my_post_object_query', 10, 3);
The only thing I need to figure out is, how to dynamically set the ID to the current user/author.