Meanwhile, I tried many many different approaches (like forcing all CC to be BCC or setting a default BCC to myself to force the field to show) but this is so far the best way:
- Create a new script called "Create new BCC message" and save it as an application:
tell application "Mail" set newMessage to make new outgoing message tell newMessage set visible to true make new bcc recipient at end of bcc recipients end tell activate end tell
- Drag this script into your Home -> Library -> Scripts folder. If the Scripts folder does not exist, create a new folder and name it Scripts.
- Now drag the "Create New BCC Message" on the dock to create a link. You could also use a script menu and even set a custom icon for the dock.

