[Andreas]

After looking over the Code for a while, I found a solution that also works for Attachments

Hook: showthread_post_start

PHP Code:

if ($forum['showfirstpost'] AND $threadedmode == 0 AND $thread['attach'])
{
    // Backup
    $thread['hasattach'] = $thread['attach'];
    unset($thread['attach']);
} 


Hook: showthread_query

PHP Code:

if ($forum['showfirstpost'])
{
    $ids .= ",$thread[firstpostid]";
    $postids = "post.postid IN (0" . $ids . ")"; 
    if ($thread['hasattach'])
    {
        $attachments = $db->query_read("
            SELECT dateline, thumbnail_dateline, filename, filesize, visible, attachmentid, counter,
                postid, IF(thumbnail_filesize > 0, 1, 0) AS hasthumbnail, thumbnail_filesize,
                attachmenttype.thumbnail AS build_thumbnail, attachmenttype.newwindow
            FROM " . TABLE_PREFIX . "attachment
            LEFT JOIN " . TABLE_PREFIX . "attachmenttype AS attachmenttype USING (extension)
            WHERE postid IN (-1" . $ids . ")
            ORDER BY attachmentid
        ");
        $postattach = array();
        while ($attachment = $db->fetch_array($attachments))
        {
            if (!$attachment['build_thumbnail'])
            {
                $attachment['hasthumbnail'] = false;
            }
            $postattach["$attachment[postid]"]["$attachment[attachmentid]"] = $attachment;
        }      
        // Restore
        $thread['attach'] = $thread['hasattach'];
        unset($thread['hasattach']);
    }
} 


What it does:
By unsetting $thread['attach'] in showthread_post_start, the original $attachments query will not be executed.
Then the Plugin on showthread_query runs this Query - with the Postid for the First Post and then restores the original Value for further processing.