Ordering results for form autocomplete fields
I would think its the query that handles the ordering. You can alter the query that fetches the results for the autocomplete field. Find the query in the module code and it should have a tag associated with it. You can then alter the query by implementing hook_query_TAG_alter

https://api.drupal.org/api/drupal/modules%21system%21system.api.php/function/hook_query_TAG_alter/7

This will allow you to change the orderby for the query

EDIT: Looking at the masquerade module I don't think it adds tags to the queries which means you cant mess with the ordering.