[TYPO3] Pre or postprocessing datamap

[Tapio Markula]

Hi

I need to my plugin button, which changes in alt_doc.php
field hide (Publish/Unpublish)

I succeed to create the button itself and it saves changes
and send a parameters, which could be used for some special tasks.

In order to force saving form fields with certain values (in my case
field 'hidden'), the most reasonable way is using pre or post processing 
using hook for class.tx_t3lib_tcemain.php


I took hook definition from another plugin and tried to use
following in my extension

require_once(t3lib_extMgm::extPath($_EXTKEY).'class.tx_t3lib_tcemain.php');
$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['t3lib/class.t3lib_tcemain.php']['processDatamapClass'][] 
= 'tx_t3lib_tcemain_process_datamap';


...
and hook file

/**
      * Hook for pre processing the incoming fieldArray
      *
      * @param    array        Incoming field array
      * @param    string        Table
      * @param    integer        UID
      * @param    object        Calling (Parent) Object
      * @return    void
      */
     function processDatamap_preProcessFieldArray(&$incomingFieldArray, 
$table, $id, $calling_obj)    {
         $myGET=t3lib_div::_GET(); // extra button should send this

         if (($table=='tt_content' || 
$table=='pages')&&is_array($incomingFieldArray) && isset($myGET['publish']))
             $incomingFieldArray['hidden']=    $myGET['publish'];
     }
doesn't do anything

I tried also
processDatamap_postProcessFieldArray
No effect.
I just don't figure why neither pre of post processing doesn't do anything.