I ended up implementing it using the JSHelper, like suggested above. I will be marking this as an answer, since it contains the actual code example on how to do it.
<h3>Search Reservations</h3>
<?php
echo $this->Html->css('screen');
// we need some javascripts for this
echo $this->Html->script('jquery');
// create the form
echo $this->Form->create(false, array('type' => 'get', 'default' => false));
echo $this->Form->input('query', array('type' => 'text','id' => 'query', 'name' => 'query', 'label' => false))?>
<div id="loading" style="display: none; ">
<?php
echo $this->Html->image('ajax_clock.gif');
?>
</div>
<?php
$this->Js->get('#query')->event('keyup', $this->Js->request(
array('controller' => 'sales','action' => 'searchReservations', $event['Event']['id']),
array(
'update' => '#view',
'async' => true,
'dataExpression' => true,
'method' => 'post',
'data' => $this->Js->serializeForm(array('isForm' => false, 'inline' => true)))
));
?>