Hi Scott, Not an easy mod, this one, as there are so many different types of messages sent by LMP.
Search admin.php for if(!$nouser){
You will see numerous if() statements that set the message body.
If you want to mod sent messages, scheduled messages, followups and the welcome email, add custom code just before this line: if($mtype=='4' && (!$conf || $xid == 'resume')){
Something like this should do it
if($mtype<'4' && !strstr($msg,'!remove')) $msg=trim($msg)."\n\n!remove";
Regards