Thanks Steve, I know how to create the hidden field. But how can it use the non-hidden text field? So I have the hidden field with the HTML version of the custom_title?
I can run a manual ‘update_post_meta’ in my sanitize callback where I store the HTML version in a meta field. But this is not ideal for serialized arrays (when I have used Group fields)

Right know I have this meta-box field

/*
Title: Intro
Post Type: projects
*/

piklist('field', array(
	'type' => 'text',
	'field' => 'project_introtitle',
	'label' => 'Project title',
	'columns' => 4,
	'sanitize'  => array(
		array(
			'type' => 'title_sanitization', // defined in functions.php (piklist-functions.php)
			'options' => 'html' // used as custom field suffix
		)
	),
	'attributes' => array(
		'placeholder' => get_the_title()
	)
));

In functions.php I have this sanitization and a bit of a hack for saving the field in a html version:

// Piklist sanitize titles (Allow forced breaks) and soft hyphens
add_filter('piklist_sanitization_rules', 'sanitize_title_function', 11);
function sanitize_title_function($sanitization_rules) {
	$sanitization_rules['title_sanitization'] = array(
		'callback' => 'sanitize_title_callback'
	);
	
	return $sanitization_rules;
}

function sanitize_title_callback($value, $field, $options) {
	$returntitle = trim(preg_replace('/\s+/', ' ', $value)); // removes any extra (unwanted) spaces
	$postid = intval( $field["object_id"] );
	$this_field = $field["field"];
	
	if (isset($options) && '' !== $options && !is_array($options)) {
		$suffix = '_' .sanitize_html_class($options);
		$htmltitle = str_replace('|', '<br />', $returntitle);
		$htmltitle = str_replace('_', '&shy;', $htmltitle);
		update_post_meta( $postid, $this_field . $suffix, $htmltitle ); // extra meta (using suffix)
	}
	return $returntitle;
}

So this is a bit hacky, and it will not work serialized arrays. So what would be the proper solution – saving to a hidden piklist field that works with groups and repeaters?

Hi Steven

I know how to create a hidden field – I just don’t know how to have one value from a piklist field saved into another (the hidden).

Field A (text) holds ‘This is my title| with a special character’

Field B (hidden) should save Field A in a HTML format ‘This is my title<br/> with a special character’