Use the D7 concept of 'Extra Fields' rather than a custom config form.

In order to give the ability to add the 'Read more' link on any display mode, anywhere in the field order, we should be using extra fields.

http://api.drupal.org/api/drupal/modules!field!field.api.php/function/ho...

This would require a new major version and probably not include an upgrade path (though that's probably okay for the small amount of manual setup that would be required).