Hi @sd
It is possible to modify a field immediately after it is loaded from the database using the acf/load_field hook. Something in the lines of this:
<?php
function my_acf_load_field( $field )
{
$field['choices'] = array(
'custom' => 'My Custom Choice'
);
return $field;
}
// acf/load_field - filter for every field
add_filter('acf/load_field', 'my_acf_load_field');