Well, I haven't solved the mystery yet, but I can already admit to one conceptual mistake: I forgot to read the source code (and relied, instead, on documentation and existing threads in the Zen Cart forums).

Also, so that I can save other beginners from the same syntax mistake, the correct syntax for accessing the $orders->field[] array elements is $orders->fields['customers_name'], not $orders->fields[customers_name] (long live single quotes...)

Fixing this syntax mistake didn't help, of course, because IMHO my problem lies within the scope/lifetime of the objects I am trying to "snatch".

In the meanwhile, I am providing here a snapshot of the my partially working solution, along with what actually gets written to the file:

includes\auto_loaders\config.writeCustInfo.php:
PHP Code:
<?php

$autoLoadConfig
[90][] = array('autoType'=>'class',
                              
'loadFile'=>'observers/class.writeCustInfo.php');
$autoLoadConfig[90][] = array('autoType'=>'classInstantiate',
                              
'className'=>'writeCustInfo',
                              
'objectName'=>'writeCustInfo');

?>
includes\classes\observers\class.writeCustInfo.php:
PHP Code:
<?php
/**
 * Observer class to write customer info to a temp file.
 */
class writeCustInfo extends base {

 
/**  constructor method !
   * 
   * Attach observer class to the global $zco_notifier and watches for a single notifier event.
   */
  
function writeCustInfo() {
    global 
$zco_notifier;
    
$zco_notifier->attach($this, array('NOTIFY_CHECKOUT_PROCESS_AFTER_SEND_ORDER_EMAIL'));
  }

 
/**   Actual Method that writes customer info to file
   * 
   * Called by observed class when any of the notifiable events occur
   *
   * @param object $class
   * @param string $eventID
   */
  
function update(&$class$eventID) {
    
$myFile "testFile.txt";
    
$fh fopen($myFile'a') or die("can't open file");

    
/**
    * Write the following fields to a file
    */
    
$stringData "This is the first line of ";
    
$stringData $stringData $myFile;
    
fwrite($fh$stringData);
    
fwrite($fh"\r\n");
    
    
$customerInfo "Customer Information: \r\n";
    
$customerInfo $customerInfo $myFile;
    
$customerInfo $customerInfo $order->fields['customers_name'];
    
$customerInfo $customerInfo $order->fields['customers_company'];
    
$customerInfo $customerInfo $order->fields['customers_street_address'];
    
fwrite($fh$customerInfo);
    
fwrite($fh"\r\n");

    
fclose($fh);    
  }
}
?>
The output, in testFile.txt:
This is the first line of testFile.txt
Customer Information:
testFile.txt
I will post here the complete solution, once I have it. But if you notice any gross mistake in my code that could explain the mystery, that would help me post the solution faster.

Thanks,
Daniel