The easiest way would be to provide your own access callback that checks the node type, e.g.
function mymodule_menu(){
$items['node/%node/register'] = array(
'page arguments' => array(1),
'access callback' => 'mymodule_node_register_tab_access',
'access arguments' => array(1),
'type' => MENU_LOCAL_TASK,
'title' => 'Register',
);
return $items;
}
function mymodule_node_register_tab_access($node) {
$valid_types = array('page');
return in_array($node->type, $valid_types);
}