I've got solution for You. I haven't got time to make an extension, but I will. Maybe even this week. You can do it manually ( which is very bad solution ) or wait. Or make extension by yourself.
Open post.php
Find (line 190)
($hook = get_hook('po_end_validation')) ? eval($hook) : null;
After paste:
$query = array(
'SELECT' => 'u.num_posts',
'FROM' => 'users AS u',
'WHERE' => 'u.id='.$forum_user['id']
);
$result = $forum_db->query_build($query) or error(__FILE__, __LINE__);
$user = $forum_db->fetch_assoc($result);
$limit = 10;
if($user['num_posts']<$limit && (preg_match('#\[url\]([^\[]*?)\[/url\]#e', $message) || preg_match('#\[url=([^\[]+?)\](.*?)\[/url\]#e', $message) || preg_match('#\[email\]([^\[]*?)\[/email\]#', $message) || preg_match('#\[email=([^\[]+?)\](.*?)\[/email\]#', $message) )) $errors[] = ('You have to write at least '.$limit.' posts to submit links.');
You can rise limit by modifying $limit variable. You can also write some other $error[] message.
If any of my dropbox link fails with 404 error, change dl.dropbox.com/u/56038890/punbb/*.zip in address to 82283017.
Currently working on rPlus - responsive theme for developers to create their own themes basing on this one.