Figured it out (with some help.)
To use the Opt-in checkbox (or you could add one if you need it) to replace the online displayed signature list “simply” add a few lines to the file “class.signaturelist.php”.
After line 56,
foreach ( $signatures as $signature ) {
add the following code,
$first_name=$signature->first_name;
$last_name=$signature->last_name;
if($signature->optin==='1'){
$first_name='Anonymous';
$last_name='';
}
That’s all, folks.