Pass variables to custom mail transport in laravel

Laravel provides the possibility to implement your own mail transport in order to send email over APIs that they aren't implemented in the framework itself by extending Illuminate\Mail\Transport\Transport. I have done so for the gmail API as this provides some benefits over sending over SMTP. There is however a scenario which is giving me some difficulties: the API requires you to impersonate a given user to send mail as that user. Currently, I'm using the 'from' address to perform that action.

use Illuminate\Mail\Transport\Transport;
use Google\Client as GoogleClient;

class MailTransport extends Transport 
{
    public function send(Swift_Mime_SimpleMessage $message, &$failedRecipients = null) 
    {
        $client = new GoogleClient;
        // initialise the client further, code omitted
        $client->setSubject(
            array_key_exists(0, array_keys($message->getFrom()))
                ? array_keys($message->getFrom())[0]
                : self::DEFAULT_SUBJECT
        );
        // send the email here using the impersonated $client
    }
}

This works fine, however I'm running into an issue when I want to send a mail from a mail alias of that user. In that case, the from address doesn't match the google account to impersonate and sending the mail doesn't work. I'm looking for a way to pass the google account to impersonate to the Mailtransport. As an instance of this class is automatically created by the framework and only Swift_Mime_SimpleMessage can be used, my best solution right now is to add a custom header to the email which is read and then removed by the MailTransport class. This feels like a dirty workaround however, does anybody know a cleaner solution?