Fossa Guard Pro V1.0.3 supports an option to hide the Subject and Recipient's name (leaving only email address) by wrapping the original message using content-type message/rfc822 and encrypting it.
New checkbox 'Hide Subject' is available once you select to Encrypt the message
The subject is hidden under generic 'Confidential Email' and looks like an empty message with encrypted attachment 'smime.p7m'
In Outlook this message is represented by a message with an internal original message S/MIME encrypted and signed
Check the video demonstrating it.
Link to the extension: Fossa Guard Pro V1.0.3