[stomph]

I got the link to the original post added in the PM by copying one line and phrase over from Andreas' hack:

1. Modify Plugin
In the plugin Quote Post in PM, find:

eval('$pm[message] = "' . fetch_template('newpost_quote', 1, 0) . '";');

After, add:

$pm['message'] = construct_phrase($vbphrase['in_reply_to_your_post_x'], $postinfo['postid'], ($postinfo['title'] ? $postinfo['title'] : $postinfo['postid'])) . "\n" . $pm['message'];

2. Add a new Phrase:
Variablename: in_reply_to_your_post_x
Text: In Reply to your Post [post={1}]{2}[/post]:

That's it!