Here's a little code snippet I never remember, and often find myself looking for in past projects. Figured it'd save me some time to paste it here.
Instantiate a model and execute some SQL (or do whatever you want) after a migraton has run:
public function after($direction) {
if ($direction == 'up') {
$Event = ClassRegistry::init('Event');
$query = "UPDATE events SET rsvp_date = early_date;";
$Event->query($query);
}
return true;
}