[kh99]

For PM, you could create a plugin using private_insertpm_process:

PHP Code:

$post1=array( 
'a','b','c' ,'d' 
); 
$post2=array( 
'e','f','g' ,'h' 
); 
$pm['message'] = str_replace($post1, $post2, $pm['message']);
$pmdm->setr('message', $pm['message']); 


I don't know about signature, I will have to take some time to figure it out.