Magento 2 - It is posible to plug-in into a _construct method?

Question

It is possible?

di.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\SalesRule\Block\Adminhtml\Promo\Quote">
        <plugin name="Vendor_Module::plugin_new" type="Vendor\Module\Plugin\New" sortOrder="1" disabled="true"/>
    </type>
</config>

New.php

namespace Vendor\Module\Plugin;

class New{
    public function before_construct(ChildBefore $subject, $interceptedInput)
    {
        echo '<pre>';
        var_dump($subject);
        var_dump($interceptedInput);
        echo '</pre>';

        return ["(before) $interceptedInput (/before)"];
    }
} 

Answer 1

No, unfortunately you can't. To overwrite _construct function, you have to use preference:

<preference for="Magento\SalesRule\Block\Adminhtml\Promo\Quote" type="Vendor\Module\Other\New" />

New.php

namespace Vendor\Module\Other;

class New extends Magento\SalesRule\Block\Adminhtml\Promo\Quote{
    protected function _construct()
    {
        echo '<pre>';
        [Your code here]
        echo '</pre>';

        return parent::_construct();
    }
} 

Answer 2

No, unfortunately plugins can only be used to to extend the behavior of any public method within a Magento class. Thus, they cannot be used to extend protected/private methods such as the _construct() method.

In order to modify those protected/private methods you will have to use preferences.

If you want to find out when and when not to use plugins I suggest you have a look at this great post: Rewriting Magento 2 classes vs Plugins