Currently extensions can custom bbcode data to bbcodes array in the function bbcode_init() in includes/message_parser.php using core.modify_bbcode_init event.
But they can't use their own validation methods like the core does:

'code'			=> array('bbcode_id' => 8,	'regexp' => array('#\[code(?:=([a-z]+))?\](.+\[/code\])#uise' => "\$this->bbcode_code('\$1', '\$2')")),

The idea is to declare $phpbb_container as a global in the function parse_bbcode() to allow extensions adding custom validation methods for bbcodes, f.e.

'code'			=> array('bbcode_id' => 8,	'regexp' => array('#\[code(?:=([a-z]+))?\](.+\[/code\])#uise' => "\$phpbb_container->get('vendor.extension.class')->custom_bbcode_validator('\$1', '\$2')")),

Also the same possibility would be logical to add to the bbcode second pass procedure (includes/bbcode.php).