You need to add a custom validation/sanitization callback, and hook it into publish_post (and/or draft_post and/or future_post, as applicable). For example:
<?php
function wpse_44807_update_custom_post_meta() {
// Globalize the $post object
global $post;
// If our custom post meta key is set, sanitize it;
// otherwise, return false
$my_post_custom = ( isset( $_POST['_my_post_custom'] ? wp_filter_nohtml_kses( $_POST['_my_post_custom'] ? false );
// Now, delete or update our custom post meta key
if ( false == $my_post_custom ) {
delete_post_meta( $post->ID, '_my_post_custom' );
} else {
update_post_meta( $post->ID, '_my_post_custom', $my_post_custom );
}
}
add_action( 'publish_post', 'wpse_44807_update_custom_post_meta' );
add_action( 'draft_post', 'wpse_44807_update_custom_post_meta' );
?>
Note that I’m sanitizing using the wp_filter_nohtml_kses() filter, which would be appropriate if you are expecting, say, a video ID or something similarly alpha-numeric. Your choice of sanitization will change, depending on the type of expected input.
Also: I’m using an underscore-prefixed custom post meta key, which is appropriate if you’re defining a custom post meta box for your custom post meta key. (The underscore prefix hides this meta key from the generic “custom field” meta box drop-down.)