<p>I'm building a Thunderbird extension and would like to add my own header to all outgoing email (e.g. &lt;myext-version: 1.0&gt; ). Any idea how to do this? I know it's possible since this is done in the OpenPGP Enigmail extension. Thanks!</p>

I don't know the answer but just some thoughts...
I think thunderbird extensions are usually just xul and js.  From the enigmail site:
Unlike most Mozilla AddOns, Enigmail
  contains platform dependent parts: it
  depends on the CPU, the compiler,
  libraries of the operating system and
  the email application it shall
  integrate into.
Looking at the Enigmail source code,
this might be the relevant section
(written in c++)
So you might need to either translate what they've done into js(!) or keep looking for a different example.
Here's another link that might be helpful