Enigmail 2.0 now supports the encryption of the email subject; however this now means that all emails from Thunderbird/Enigmail to EM Client have the subject “Encrypted Message” and any message has to be opened in an external application.
From the GPG forums I see that the implementation is described here:
https://github.com/autocrypt/memoryhole/blob/master/specs/draft-memoryhole.md
Having this ability would enable me to get off Thunderbird, and hopefully encourage my company to do likewise