I was looking how to hack thunderbird to add an extra header when I compose a new mail.
I started this with some success only to display it but not yet so add the new field in the header when the mail is sent.
Then I found mnenhy (1), this is a very nice project that allows me to add what I want… but I’d like to link that new extra field to an openLDAP and this is not yet possible… If I’ve time I’ll try to fix this.