Dug in a bit. That will indeed add that phrasegroup to the array which ini's $vbphrase, however, for some odd reason, at that moment $vbulletin->userinfo contains elements "phrasegroup_x" full of serialized array values which are unset after $vbphrase is created. This is called in fetch_userinfo which is called includes/init.php. This all happens before that hook.
Hook: fetch_userinfo_query
PHP Code:
global $phrasegroups;
$phrasegroups[] = 'posting';
I honestly think there's got to be a better way. I don't like hooking into there and I don't think it would work unless you're logged in. I got sick of digging though, I'll probably come back to it later.