I tried this very thing. I discovered you cannot effect a change in /etc/inittab until after tc-config exits because it is run by init. Therefore there is no opportunity to prevent auto-login via this method unless you remaster. However, since extensions are processed before the autologin you have the opportunity to modify /root/.profile.
I created an extension that saves the current /root/.profile and replaces it with one which will:
1) Copy /etc/inittab-save over /etc/inittab (you may wish to copy a custom one or otherwise modify the original /etc/inittab)
2) Reload init (kill -SIGHUP 1)
3) Restore the original /root/.profile
4) Logout (via exit)
The autologin processes this new /root/.profile. It reloads the desired /etc/inittab and logs out. Thus, autologin never occurs more than once, and the user is never presented with a shell. The TC developers could modify the stock /root/.profile to detect a boot parameter and execute logic that effects the same changes.
With some other extensions to enable NIS, LDAP, or create local users, this can enable a conventional multi-user system. My extension contains two files:
usr/local/tce.installed/noautologin
mv /root/.profile /root/.profile.orig
mv /root/.profile.noautologin /root/.profile
root/.profile.noautologin
#!/bin/sh
if [ -e /root/.profile.orig ]; then
cp /etc/inittab-save /etc/inittab
kill -SIGHUP 1
mv /root/.profile.orig /root/.profile
exit
fi