Encrypt Lightning event invite mails with pgp

Question

I use Thunderbirds/Icedoves Lightning add-on to manage my calendar, tasks and notes.

When creating an event I can add attendees and send invitations to them, but they won't use their public pgp keys to encrypt the invitation.

How can I force the invitations to be encrypted?

edit: as suggested by @schroeder, I've asked the question on the mozilla support page. For anyone interested, here's the link.