here’s how I did it:

function theme_external_link($url, $content = null) {
    //Long URL functionality.  Also uncomment function long_url($shortURL)
    if (!$content)
    {
        $longurl = long_url($url);
        $domain = parse_url(long_url($url));
        if (setting_fetch('gwt') == 'on') {
            $encoded = urlencode($longurl);
            $longurl = "http://google.com/gwt/n?u=$encoded";
        }
        return "<a href='$longurl' target='_blank'>[".$domain["host"]."]</a>";
    }
    else
    {
        return "<a href='$url' target='_blank'>$content</a>";
    }
}

I also moved the GWT modifications there since it doesn’t handle all shortened urls well, plus I still wanted to get more URL context via the full link if I mousedover it

hope the code pastes in here correctly