Magento – Adding custom column into product grid

I have created a custom column for associated products (basically every simple product will have that column under "associated products" tab of configurable product). I store information of this field/column into an attribute which belong to configurable product instead of each simple product.

Below is the code for adding a column;

config.xml

<core_block_abstract_to_html_before>
     <observers>
        <something>
          <class>namespace_JsonProductInfo_Model_Observer</class>
          <method>addPositionColumn</method>
        </something>
     </observers>
</core_block_abstract_to_html_before>

Oberver.php

public function addPositionColumn(Varien_Event_Observer $observer)
{
    $block = $observer->getEvent()->getBlock();
    if ($block instanceof Mage_Adminhtml_Block_Catalog_Product_Edit_Tab_Super_Config_Grid) {
        if (!$block->isReadonly()) {
            $block->addColumnAfter(
                'position_simple',
                array(
                    'header'     => Mage::helper('jsonproductinfo')->__('Position'),
                    'type'      => 'input',
                    'name'      => 'position_simple',
                    'index'     => 'position_simple',
                    'sortable'  => false,
                ),
                'name'
            );
        }
    }
}

With above code I am able to add the column into a require grid. And I am also able to save the information which I put inside that column (which is basically an attribute). The only problem I am facing is, I am unable to display the stored information into that column again (If I edit the product). I have also created another observer of type eav_collection_abstract_load_befor but no luck so far.
Below is the code for it:

config.xml

<eav_collection_abstract_load_before>
    <observers>
        <jsonproductinfo>
            <class>namespace_JsonProductInfo_Model_Observer</class>
            <method>addPositionToCatalogProductCollection</method>
        </jsonproductinfo>
    </observers>
</eav_collection_abstract_load_before>

Observer.php

public function addPositionToCatalogProductCollection($observer)
{
    $collection = $observer->getEvent()->getCollection();
    if (!isset($collection)) {
        return;
    }
    $collection->addAttributeToSelect('position_simple');
}

I would be grateful if someone can point to the mistake or guide me how can I display back that information again into that column.

EDIT

And here is the code which creates the "postion_simple".

$this->addAttribute(
    'catalog_product',
    'position_simple',
    array(
        'group'             => 'General',
        'type'              => 'varchar',
        'input'             => 'hidden',
        'backend'           => '',
        'frontend'          => '',
        'label'             => 'Simple Position',
        'class'             => '',
        'source'            => '',
        'global'            => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_GLOBAL,
        'visible'           => true,
        'required'          => false,
        'user_defined'      => true,
        'default'           => '',
        'searchable'        => false,
        'filterable'        => false,
        'comparable'        => false,
        'visible_on_front'   => false,
        'visible_in_advanced_search'   => false,
        'unique'            => false,
        'apply_to'          => Mage_Catalog_Model_Product_Type::TYPE_CONFIGURABLE,
        'is_configurable'   => false,
    )
);