[Exim] Log "Envelope-to:" in system filter

I have a system filter writing a custom log file. I would like to add
Envelope-to to the log file, but cannot work out how to do this.

$h_envelope-to isn't filled because the header hasn't been added by a
transport yet.

Thanks,
Rory

--
Rory Campbell-Lange
<rory@???>
<www.campbell-lange.net>