Nope ... bash is just a shell. Login is its own program, /bin/login. On Linux systems /bin/login tends to be part of the util-linux package. You can find the source for various util-linux version at
kernel.org. In your case though, it might be easier just to write a new PAM (pluggable authentication module) to do what you want to do, and then modify the config files in /etc/pam.d (or the /etc/pam.conf file on some systems) to do authentication using your module. Seems cleaner than hacking /bin/login to me, at least.
edited to add: I did a little more googling, and some distros put /bin/login in part of the shadow utils package.
