Enrich Magento product edit admin form with own model data

form_own_model_data.php

<?php

    /**
     * In some Mage_Adminhtml_Block_Catalog_Form class...
     */
    protected function _prepareForm()
    {
        $formDataObject = new Varien_Object() // Your logic to get original data for your form

        $form = new Varien_Data_Form();

        $fieldset = $form->addFieldset('my_fieldset', array('legend'=>Mage::helper('my_helper')->__('My Legend'), 'class'=>'fieldset-wide'));

        $fieldset->addField('my_form_element_id', 'text', array(
            'name'      => 'my_form_element_id',
            'label'     => Mage::helper('my_helper')->__('Some Label'),
            'title'     => Mage::helper('my_helper')->__('Some Title'),
        ));

        $form = $this->_updateFormData($formDataObject, $form);

        $form->setValues($formDataObject->getData());
        $this->setForm($form);

        Mage::dispatchEvent('my_nice_event_name', array('form' => $form));

        return parent::_prepareForm();
    }

    /**
     * Update / enrich form with your own model data
     *
     * @param Varien_Object $formDataObject // Data of the form as a Varien_Object
     * @param Varien_Data_Form $form
     * @return Varien_Data_Form
     */
    protected function _updateFormData($formDataObject, $form)
    {
        if($formDataObject) {
            $model = new Varien_Object() // Your logic to get your model for your business use;
            $model->setAttributeDefaultValue('my_fake_attribute_code', 'my_default_value');

            // Prepare data for displaying "Use default" checkbox
            if($model->getStoreId() == Mage_Core_Model_App::ADMIN_STORE_ID) {
                $model->setStoreId($formDataObject->getStoreId());
            }
            $form->setDataObject($model);

            // If model instance has data, update form original data
            if($model->hasData()) {
                $formDataObject->setData('my_form_element_id', $model->getData('my_data_key'));
            }
        }

        // Create dummy attribute instance for displaying "Use default" checkbox
        $attribute = Mage::getResourceModel('catalog/eav_attribute');
        $attribute->setData('is_global', false);
        $attribute->setAttributeCode('my_fake_attribute_code');
        $form->getElement('my_form_element_id')->setEntityAttribute($attribute);

        return $form;
    }