Since you have already created the attribute, you can add some layout XML to inject a helper script:
<adminhtml_catalog_product_edit>
<reference name="head">
<action method="addJs">
<script>path/to/your/script.js</script>
</action>
</reference>
</adminhtml_catalog_product_edit>
For the product edit area only. You can place this in your related module's layout XML or else in app/design/adminhtml/default/default/layout/local.xml. So in the JS, I presume you'll be observing the change event of the target select element. On trigger, you'll make an AJAX call to ... ? I don't know that either, but if it's going to route back through Magento to get that information, you will likely need to also setup a controller.
Alternate Method
Another way to get the JS in place is to specify an "input renderer" when installing your attribute. Here's an article on how to do that: http://inchoo.net/magento/adding-magento-attribute-with-custom-input-renderer/
To summarize the article, install your attribute like so:
..
$installer->addAttribute(Mage_Catalog_Model_Product::ENTITY, 'example_field', array(
'group' => 'General',
'type' => 'text',
'backend' => '',
'input_renderer' => 'test/catalog_product_helper_form_example',
..
Notice the input_renderer field. This maps to a block which renders the form field for your attribute, like so:
class Namespace_Test_Block_Catalog_Product_Helper_Form_Example
extends Varien_Data_Form_Element_Text
{
public function getAfterElementHtml()
{
$html = parent::getAfterElementHtml();
return $html . "<script>//your script here</script>";
}
...
You might not want to do this because your attribute is already installed, but I tend to prefer this method because of how tightly it couples the JS with the attribute.
