if ($active == status window) ...
You can check if the $cid of the new active status is higher than the last opened $cid. Then you know if it's a newly openened window. You then save the value of this $cid as last one, and so on.
Aren't you the author of a full script, with all sorts of whistles and bells? This should be a walk in the park...
EDIT:
First type //set %lastcid $scon($scon(0)).cid
on *:active:*:{
if ($active == status window) && ($cid > %lastcid) {
inc %lastcid
echo -a New status window: $status
}
}
on *:start: set %lastcid 1