Which hook do I use to alter product entity data in a product display?
hook_entity_load may be the answer that some folks are looking for in situations that differ from mine.

For my specific need, I discovered that everything I needed to accomplish could be done with hook_form_alter.  The form_state holds the value of the currently selected SKU in $form_state['default_product']->sku.  Here is a hook_form_alter function I wrote to accomplish my goals:

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    if(strstr($form_id, 'commerce_cart_add_to_cart_form_')) {

        // Load the current product entity
        $prod = commerce_product_load_by_sku($form_state['default_product']-&gt;sku);

        // Check and see if the produt is customizeable or not, and then react
        $customizeable = $prod-&gt;field_prod_customizable['und'][0]['value'];
        if(isset($customizeable) &amp;&amp; $customizeable == 1) {
            // Grab the custom label and instruction
            $customize_label = $prod-&gt;field_prod_customize_label['und'][0]['value'];
            $customize_instruction = $prod-&gt;field_prod_customize_instruction['und'][0]['value'];

            // Alter the field to display the custom label and instruction
            $form['line_item_fields']['field_prod_comments']['und'][0]['value']['#title'] = $customize_label;
            $form['line_item_fields']['field_prod_comments']['und'][0]['value']['#description'] = $customize_instruction;
        }
        else {
            // If not a customizeable product, then make the field hidden and not required
            $form['line_item_fields']['field_prod_comments']['und'][0]['value']['#type'] = 'hidden';
            $form['line_item_fields']['field_prod_comments']['und'][0]['value']['#required'] = 0;
        }
    }
}


Note that the textarea field I added to my Product line item type is called field_prod_comments.  The fields I added to my product entity are field_prod_customizable, field_prod_customize_label, and field_prod_customize_instruction.