Preparations (in IDE)

1. open the themes/advanced/error_notfound.tpl file for editing
2. add below shown code after <inp2:m_phrase name="lu_text_PageNotFound"/> tag:

<inp2:m_DefineElement name="product_element">
    [<inp2:Field name="SKU"/>]
</inp2:m_DefineElement>
<inp2:p_PrintList render_as="product_element"/>

3. save changes
4. open the themes/advanced/platform/my_account/my_account.tpl file for editing
5. add below shown code after the topmost HTML comment on the template:

<?php
    $application =& kApplication::Instance();
    $application->UrlManager->show404();
?>

6. save changes

Actual testing

1. go to the Front-End
2. confirm, that you're setting an expected page
3. open /missing/500/ page (manually append to base URL of the website)
4. confirm, that Page Not Found page was shown
5. click on the My Account link in the header
6. confirm, that Page Not Found page was shown
7. click on the Register link in the sidebar
8. confirm, that you're setting an expected page