hook run just in message-mode just before user begins composing/replying?

[Gregor Zattler]

I wrote two functions which, manually called in a
buffer just set up by message-mode for me to start
typing my message/reply, manipulate the headers of said
message.

ATM I call them in message-send-hook but then I do not
see the result of the manipulations and sometimes I would
like to adjust headers after these functions run.

I did not find a hook for this purpose.  I experimented
with message-signature-setup-hook but then one function
mangles the headers, the other function does nothing,
indicating that the headers are not setup or not
accessible as at the moment the user (I) starts to
type.

Any ideas how to solve this puzzle?