Server side validation

You can capture the form submission by creating a small plugin / or by editing your theme functions file, and you can do this using a ajax hook. In the plugin you would load this on the edit post page:

jQuery(document).ready(function(){

jQuery('#post').submit(function(){
        var request = jQuery(this).serializeArray();
        request.push({name: 'action', value: 'check_some_stuff'});

        jQuery.post(ajaxurl, request, function(response){
            if(response.error){
                response = jQuery.parseJSON(response);
            jQuery('#local-storage-notice').after('<div class="error">'+response.error+'</div>');
        });
                return false;
        } else {
            return true;
        }
    });

});

When you return false, it will stop the form from submission.

The ajax will call a function check_some_stuff. The request gets send as $_POST. You can then validate server side all you want:

add_action( 'wp_ajax_check_some_stuff', 'check_some_stuff' );

public function check_some_stuff()
{
    //validate some stuff here
    $valid = false;
    if($valid){
        $results = array('msg'=>'You win');
        print json_encode($results);
    } else {
        $results = array('error'=>'<pre>'+print_r($_POST, true)+'</pre>');
        print json_encode($results);
    }
    die();
}

I can go in more detail on how to add the javascript in if you need.