<?php

function _meta_sanitize_cb ( $meta_value, $meta_key, $meta_type ) {
	return $meta_value;
}

function _meta_yes_you_can( $can, $key, $post_id, $user_id, $cap, $caps ) {
	return true;
}

function test_register_meta() {
	register_meta( 'post', '_foo', '_meta_sanitize_cb', '_meta_yes_you_can');
}
add_action( 'init', 'test_register_meta' );