[Evolution] Include OpenPGP headers in outgoing mails

[Ju Nique <j-unique outlook com>]

Hello,

I'm using Evolution 3.30.3 with gpg and want to send the Enigmail OpenPGP headers 
[https://www.enigmail.net/index.php/en/user-manual/signature-and-encryption "The first two options add a new 
mail header to the message..."] along with signed mails. There are multiple accounts with different key IDs.

What's the best way to achieve that? I have thought about

* the Custom Header Lines 
[https://help.gnome.org/users/evolution/stable/mail-composer-custom-header-lines.html.ru] plugin, but it 
seems that it doesn't have any automation, so I'd have to set the header for every new email, and I'd surely 
forget it most often
* using rules for outgoing mails, but there is neither a "Add header" nor a "process mail by external 
program" action
* writing a separate plugin - but I think it should be integrated into the core OpenPGP support
* patching Evolution (how do I find out if Evolution developers would accept this feature?)
* setting these headers server-side when a specific From header is given (last resort)

Do you have a suggestion?

Best regards,
J.