Use a template with the BCC field filled in.
Poco 3 :
If you want to use this template automatically, you can select it as the
default at the Account level or for specific people in your address book.
Select your preferred template in the Templates tab when you set up accounts or
Poco 2 :
This won't work for replies unless you reply using a template and you will always have to use the "New message
from template" command when starting a new message.
(Note: If there are undocumented command tags that let you issue the
"New message from template" and "Reply with template" commands then you could create a
skin that would automate this functionality for the "New" and "Reply"
buttons or add separate buttons for those tasks). I'm not sure if these tags exist.