Temporarily solved it wthis way:
/**
* Maybe add required span to labels
*/
add_filter( 'rwmb_begin_html', function ( $begin, $field, $meta ) {
if ( is_admin() )
return $begin;
if ( ! empty ( $field['required'] ) ) {
$begin = str_replace( '</label>', '<span class="req">*</span></label>', $begin );
}
return $begin;
}, 10, 3 );
Additional question: Tried to apply this hook only to the frontend submission fields, by using filters/prefixes like rwmb_frontend_begin_html, mb_frontend_begin_html, rwmb_mb_frontend_begin_html but none of them worked.
.
