This is a super old question, but obviously got quite a few views. My curiosity drowned, I thought I would look. After some involvement, I found that you can add the following header to make the message editable:
X-Uniform-Type-Identifier: com.apple.mail-draft
Add this to your RFC-2822 compatible message, give it the extension .eml or .emlx , and when you double-click it will open as an editable message in Mail.app.
Good luck.
angrychimp
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