[Xenon]

no it's not possible, without rewriting more scripts...

the lastpost is saved from within newreply.php
you can try to replace in newreply.php
this line:

PHP Code:

$DB_site->query("UPDATE forum SET replycount=replycount+1,lastpost='".time()."',lastposter='".addslashes($postusername)."' WHERE forumid IN ($foruminfo[parentlist])"); 


with this one

PHP Code:

$DB_site->query("UPDATE forum SET replycount=replycount+1,lastpost='".time()."',lastposter='".addslashes($postusername)."' WHERE forumid=$threadinfo[forumid]"); 


but i can't prove it'll work perfect