Magento – How to set default value of custom options for product

Your question is a bit confusing.

What you are saying/asking is how to add the field to 'info_buy_request' options, if it was not filled in by a user.

If the user did not fill it in, what exactly do you want to add? The field with NULL as the value? If the field does not exist in the options, then that explicitly means it was not populated, which should be enough, should it not? Why the need to populate it? (and again, with what?)

also, your way of iterating over the item options is not needed. You can get to the data as follows:

$infoBuyRequest = $item->getOptionByCode('info_buyRequest'); 
if (is_object($infoBuyRequest)) { 
    $buyRequest = new Varien_Object(unserialize($infoBuyRequest->getValue())); 
} 

Now your data is in an object - $buyRequest and can be accessed/manipulated as an object

I don't know if recommending an extension counts as a valid answer (for a question with a bounty) but someone claims here that he wrote an extension that does exactly what you need. Extension can be downloaded from here. I haven't tested it but, as soon as I do, I will post and update. I only looked through the code and there is a lot of it. I lost interest after 2 files. I hope it works for you.

[EDIT]
I've got my interest back. I tested the extension on ce-1.7.0.2 and it (almost) works.
If you use it with developer mode on you will get some errors. Here is what you need to change to make it work.
in /app/code/local/Magebuzz/Customoption/controllers/Adminhtml/CustomoptionController.php on line 28 there is this:

$model->setData('value['.$option_id.']',$value[0]['option_type_id']);

This shows an 'undefined index' warning. To avoid it, wrap it in an if statement.

if (isset($value[0])){
    $model->setData('value['.$option_id.']',$value[0]['option_type_id']);
}

In /app/code/local/Magebuzz/Customoption/Block/Adminhtml/Customoption/Edit/Tab/Form.php on line 129 there is this:

foreach ($values as $value) {
    $valuesArr[$value['option_type_id']]=$value['title'];
}

$values may be null so change the code to this:

if (is_array($values)){
    foreach ($values as $value) {
        $valuesArr[$value['option_type_id']]=$value['title'];
    }
}

Now it should work.
Now some review.

1. PRO. Works perfectly for dropdown and radio custom options
2. Neurtal. Kind of works for multiple select and checkbox custom options. You can select only one value for the available values.
3. Inconvenient but I can overlook it. in order to set the default values you have to click on an other menu item in the backend and look for your product in the list.
4. Con. It doesn't work for text, textarea, date, datetime, time, file custom options. But with a little work it can be changed to work correctly for all types (maybe except file).