Meta Box

It turns out the values can be easily filtered using the ´rwmb_value´ filter like so:

add_filter( 'rwmb_value', array( $this, 'uncheck_thruthiness' ), 20, 3 );
public function uncheck_thruthiness ( $new, $field, $old ) {
	if($field['id'] == 'truthiness' && !empty( $field['attributes']['mod_lock'] ) ){
		$new = '';
	}
        return $new;
}

I have a different issue now... This can be marked as resolved. I'll open a new thread.

Here are the full selectomatic custom field extended classes based on the Select field:

if ( class_exists( 'RWMB_Field' ) ) {
	class RWMB_Selectomatic_Field extends RWMB_Select_Field {
		public static function walk( $field, $options, $db_fields, $meta ) {
			$attributes = self::call( 'get_attributes', $field, $meta );
			$walker     = new RWMB_Walker_Selectomatic( $db_fields, $field, $meta );
			$output     = sprintf(
				'<select %s>',
				self::render_attributes( $attributes )
			);
			if ( false === $field['multiple'] ) {
				$output .= $field['placeholder'] ? '<option value="">' . esc_html( $field['placeholder'] ) . '</option>' : '';
			}
			$output .= $walker->walk( $options, $field['flatten'] ? - 1 : 0 );
			$output .= '</select>';
			$output .= self::get_select_all_html( $field );
			return $output;
		}
	}

	class RWMB_Walker_Selectomatic extends RWMB_Walker_Select {

		public function start_el( &$output, $object, $depth = 0, $args = array(), $current_object_id = 0 ) {
			$label  = $this->db_fields['label'];
			$id     = $this->db_fields['id'];
			$meta   = $this->meta;

			if($depth){
				$output .= sprintf(
					'<option value="%s" %s>%s</option>',
					esc_attr( $object->$id ),
					selected( in_array( $object->$id, $meta ), true, false ),
					esc_html( RWMB_Field::filter( 'choice_label', $object->$label, $this->field, $object )
					)
				);
			}
			else{
				$output .= sprintf(
					'<optgroup label="%s">',
					esc_html( RWMB_Field::filter( 'choice_label', $object->$label, $this->field, $object )
					)
				);
			}
		}

		public function rwmb_end_html_el( &$output, $object, $depth = 0, $args = array(), $current_object_id = 0 ) {
			if(!$depth){
				$output .= '</optgroup>';
			}
		}
	}
}

I was able to achieve something building off of the standard select field. I created a new field type called: 'selectomatic' which is defined as so:

array(
    'type'    => 'selectomatic',
    'id'      => 'animal_select',
    'name'    => 'Animals',
    'options' => array(
        array( 'value' => 'monkey', 'label' => 'Monkeys' ),
            array( 'value' => 'king_kong', 'label' => 'King Kong', 'parent' => 'monkey' ),
            array( 'value' => 'curious_george', 'label' => 'Curious George', 'parent' => 'monkey' ),
        array( 'value' => 'donkey', 'label' => 'Donkeys' ),
            array( 'value' => 'eeyore', 'label' => 'Eeyore', 'parent' => 'donkey' ),
            array( 'value' => 'guss', 'label' => 'Gus', 'parent' => 'donkey' ),
    ),
    'flatten' => false
),

Then I extended the Select Field Class and Select Walker Class like so:

class RWMB_Walker_Selectomatic extends RWMB_Walker_Select {

		public function start_el( &$output, $object, $depth = 0, $args = array(), $current_object_id = 0 ) {
			$label  = $this->db_fields['label'];
			$id     = $this->db_fields['id'];
			$meta   = $this->meta;
			
			if($depth){
				$output .= sprintf(
					'<option value="%s" %s>%s</option>',
					esc_attr( $object->$id ),
					selected( in_array( $object->$id, $meta ), true, false ),
					esc_html( RWMB_Field::filter( 'choice_label', $object->$label, $this->field, $object )
					)
				);
			}
			else{
				$output .= sprintf(
					'<optgroup label="%s">',
					esc_html( RWMB_Field::filter( 'choice_label', $object->$label, $this->field, $object )
					)
				);
			}
		}

		public function rwmb_end_html_el( &$output, $object, $depth = 0, $args = array(), $current_object_id = 0 ) {
			if(!$depth){
				$output .= '</optgroup>';
			}
		}
	}

It's not beautiful, but it's a work-around that... works!

Thank you for fixing the link in the initial post. Should read:
There is currently no support for the <optgroup> tag in the Select and Select-Advanced field types...

Anh, that worked perfectly! Thank you, thank you, thank you. You can close this issue as resolved. But please don't forget to add this to the documentation... it will save the next person some grey hairs :D.

At this point it seems the only way currently to 'filter' the save process using a fields custom attributes would be to create a series of new custom fields types. These new 'extended' field types would only serve to overwrite the static save method as shown above. There must be a better way to do this.

This is what I ended up doing:
I include the meta-box file only if the plugin is activated like so:

if ( is_plugin_active( 'tmeta-box-group/meta-box-group.php' ) ){
	include ('inc/config-meta-boxes.php');
}

NOTE: the folder is now tmeta-box-group (not sure when this changed) and NOT meta-box-group.