[calorie]

An idea... Look in newattachment.php for the following:

if ($threadid) // newreply.php or editpost.php called
{
	// blah blah
}
else if (!($forumperms & $vbulletin->bf_ugp_forumpermissions['canpostnew'])) // newthread.php
{
	print_no_permission();
}

And add something after that, perhaps like follows:

if (!$threadid AND ($forumperms & $vbulletin->bf_ugp_forumpermissions['canpostnew']))
{
	// do your stuff
}