Product load event - regardless of context

You don't need an observer to do this. You can use the attribute backend_model to handle your logic.

If you specify a backend_model with your attribute and you extend the Mage_Eav_Model_Entity_Attribute_Backend_Abstract class within your backend model class you can use the afterLoad, beforeSave, afterSave, beforeDelete or afterDelete methods to execute your own logic.

Using the backend_model is the correct way to implement this sort of logic and is how native Magento attributes do it where they have similiar requirements.