If I understand you correctly, you want to have JS specific to a page loading in the header when you call a certain element, but that the JS could be different for each element. You also want the JS to be referenced at the beginning of your HTML document.
This is actually quite easy to do. Just make sure that you have <?php echo $scripts_for_layout; ?> in you <head> tag in the layout you are using.
Then, within the element, just do:
<?php $this->Html->script("js_file", array("inline"=>false)); ?>
js_file is the name of your JavaScript file in app/webroot/js/. In this case, the file would be called js_file.js but you must leave off the .js when referencing it as above.
It doesn't matter where abouts in the element file you put this because the "inline"=>false part ensures it won't actually appear at that stage in the code. Instead, it will appear in the <head> wherever you put <?php echo $scripts_for_layout; ?> in your layout.