Answer remade. The original was a wild idea...
The solution is the same as the one posted by Rarst in the question linked
How to *remove* a parent theme page template from a child theme?
Difference being the
And a check for only running in
edit-page and not in
edit-custom_post_type, as all these cases are fired by
The parent is "Twenty Eleven" and the child is called "Twenty Twelve".
if('edit-page' != $current_screen->id)
$templates = &$wp_themes['Twenty Twelve']['Template Files'];
$template1 = trailingslashit( TEMPLATEPATH ).'showcase.php';
$template2 = trailingslashit( TEMPLATEPATH ).'sidebar-page.php';
$key1 = array_search($template1, $templates);
$key2 = array_search($template2, $templates);
unset( $templates[$key1], $templates[$key2] );