No support for %tag% tag

It seems get_permalink doesn’t support the documented %tag% tag. Here’s an excerpt from the defined code:

function get_permalink($id = 0) {
    $rewritecode = array(
        '%year%',
        '%monthnum%',
        '%day%',
        '%hour%',
        '%minute%',
        '%second%',
        '%postname%',
        '%post_id%',
        '%category%',
        '%author%',
        '%pagename%'
    );

I would really like my custom permalink to be:
/posts/%tag%/%postname%.html

Any thoughts on an alternative way to achieve this? Or do I need to start hacking some code?

Thanks!
– Lon