Display decrypted or empty value in configuration

Question

I have a configuration value that I encrypt upon save, but when I look at it again in the configuration (UI), I get the encrypted string. I would like to show an empty string (production) or the correct decrypted value (for testing), but cannot quite figure out how to do this.

The logical way I know of is to write a source model, but those seem to only work for select elements. This is a simple text field. I need a way to receive the original data, decrypt with (with Magento\Framework\Encryption\EncryptorInterface), and display the decrypted value; either that or an empty value.

Answer 1

Partial answer:

Turns out, in this specific case, it's possible to have Magento decrypt the value automatically on display / retrieval, by setting a backend model in config.xml too. For example:

<section>
    <group>
        <field backend_model="Magento\Config\Model\Config\Backend\Encrypted" />
    </group>
</section>

However, it does not answer the question's general case (especially the blank value), an answer addressing this would be appreciated!