Magento – When to add custom data to quote

I would try a slightly different approach. Instead of adding your own field you can use Magento's product options. This will be saved as part of the info_buyRequest and will be part of a reorder as well.

I am currently not sure when you save your comments. The product option needs to be added before the product is added to the cart.

This event might help:

    $eventName = sprintf('catalog_product_type_prepare_%s_options', $processMode);
    Mage::dispatchEvent($eventName, array(
        'transport'   => $transport,
        'buy_request' => $buyRequest,
        'product' => $product
    ));

with $processMode='full'; you can then add your comment as a custom option:

$product->addCustomOption('fooman_product_comment', 'The Comment!'); 

Something to consider - The product option will automatically be displayed everywhere.

Sure, the quote_id is saved with the order, so you can just:

$order->getQuoteId()

And the method should work too, maybe you couldn't load the order...

To get the order, you can hook in one of the events after the order creation:

• checkout_onepage_controller_success_action
• checkout_type_onepage_save_order_after
• checkout_submit_all_after

If you only need the order ID, you can just use checkout_type_onepage_save_order_after and save the ID to the database.