hgrc.5: whether bcc is prompted for is up to the extension
There is no code in mail.py that forbids an extension to prompt for bcc.
#
# tcsh completion for Mercurial
#
# This file has been auto-generated by tcsh_completion_build.sh for
# Mercurial Distributed SCM (version 1.3.1+269-5d8125bbbbf4)
#
# Copyright (C) 2005 TK Soh.
#
# This is free software; you can redistribute it and/or modify it under
# the terms of the GNU General Public License as published by the Free
# Software Foundation; either version 2 of the License, or (at your
# option) any later version.
#
complete hg \
'n/--cwd/d/' 'n/-R/d/' 'n/--repository/d/' \
'C/-/( -R --repository \
--cwd \
-y --noninteractive \
-q --quiet \
-v --verbose \
--config \
--debug \
--debugger \
--encoding \
--encodingmode \
--traceback \
--time \
--profile \
--version \
-h --help)/' \
'p/1/(add addremove annotate blame archive \
backout bisect branch branches bundle \
cat clone commit ci copy \
cp debugancestor debugcheckstate debugcommands debugcomplete \
debugdata debugdate debugfsinfo debugindex debugindexdot \
debuginstall debugrebuildstate debugrename debugsetparents debugstate \
debugsub debugwalk diff export forget \
grep heads help identify id \
import patch incoming in init \
locate log history manifest merge \
outgoing out parents paths pull \
push recover remove rm rename \
mv resolve revert rollback root \
serve showconfig debugconfig status st \
summary sum tag tags tip \
unbundle update up checkout co \
verify version)/'