Dynamic processing of string field based on user IP
Store the data in the cleanest way possible and then use something like Custom Formatters
or a custom module to modify the presentation of the stored data.

As for using $_SERVER['REMOTE_ADDR'] I feel that I need more data on how and where this will be used.  Have you considered using the permissions or roles built into Drupal?

You would likely need to place this in a preprocess function to conditionally call it for a specific IP address.

For example.

/**
 * Implements hook_preprocess_node().
 */
function hook_preprocess_node(&amp;$variables) {
    //Obviously a real IP is required above for correct syntax.
    if ($_SERVER['REMOTE_ADDR'] == xxx.xxx.xxx.xxx) {
        //Next you can edit the output of any field you want if your remote address matches.
        $variables['field_myfield'][LANGUAGE_NONE][0]['value'] = 'Something Modified Here';
    }
}