How do I edit the response of successful register.json?
I was never able to get back the raw text password without hacking on the logintoboggan / user modules themselves (similar to what is shown in my question).  

However, I solved my same purpose of providing immediate-login-upon-registration for Services/API calls using the services_token_access module. This code only hooks onto a success registration then ads the services_token to the response which is now the only thing my client now needs for validation.

function HOOK_services_request_postprocess_alter($controller, $args, &amp;$result) {

if ( gettype($result) == 'array' &amp;&amp; isset($result['uri']) &amp;&amp; isset($result['uid'])) { // THIS CONDITION MATCHES THE DEFAULT RESPONSE OF A SUCCESSFUL REGISTRATION. 
        $user = (object)$result;
        $user = user_load($user-&gt;uid);
        $user = HOOK_services_account_object_alter($user);
        $result = array('token'=&gt;$user-&gt;services_token, 'user'=&gt;$user);
    }
    return $result;
}


function HOOK_services_account_object_alter(&amp;$user) {
    $user-&gt;services_token = services_token_access_update_token($user-&gt;uid, FALSE); // essential for logging in later
    $user-&gt;specialstuff = 'whatever else you need';
    return $user;
}


(The services_token_access module is not particularly popular and barely supported, but worked for me.)