Idea for new register meta functionality in core.

gistfile1.php

<?php

$_wp_registered_meta = array();

/**
 * Register meta key
 *
 * Supported arguments.
 * label, field_type, sanitize_callback, auth_callback
 *
 * @since 3.3.0
 *
 * @param string $meta_type Type of meta
 * @param string $meta_key Meta key
 * @param array $args Arguments
 */
function register_meta( $meta_type, $meta_key, $args = array() ) {

	global $_wp_registered_meta;

	$args = wp_parse_args( $args, array(
		'label'      => $meta_key,
		'field_type' => 'text',
	) );

	$_wp_registered_meta[$meta_type][$meta_key] = $args;

	if ( isset( $args['sanitize_callback'] ) && is_callable( $args['sanitize_callback'] ) )
		add_filter( "sanitize_{$meta_type}_meta_{$meta_key}", $args['sanitize_callback'], 10, 3 );

	if ( empty( $args['auth_callback'] ) ) {
		if ( is_protected_meta( $meta_key, $meta_type ) )
			$args['auth_callback'] = '__return_false';
		else
			$args['auth_callback'] = '__return_true';
	}

	if ( is_callable( $args['auth_callback'] ) ) {
		add_filter( "auth_{$meta_type}_meta_{$meta_key}", $args['auth_callback'], 10, 6 );
	}

}

/**
 * Get all registered meta.
 *
 * @return array
 */
function get_registered_meta( $type = null ) {

	global $_wp_registered_meta;

	if ( ! $_wp_registered_meta ) {
		return array();
	}

	if ( $type ) {

		if ( isset( $_wp_registered_meta[ $type ] ) ) {
			return $_wp_registered_meta[ $type ];
		} else {
			return array();
		}

	}

	return $_wp_registered_meta;

}

• I have changed the args required by the register_meta function - although we could work with the old args for backwards compat if neccesary.