I realize that this is really old, but there are no answers here.
I just wrote a post on how to set up Squid with seamless auth using basic_ncsa_auth and htpasswd. I know that this post asks for an LDAP solution, but it's always nice to have alternatives, especially when they are so easy to set up. I hope this comes in handy.
Here's the full post.
and here's a quick link
to making the auth seamless.
Essentially you just set up a user name and password combination for regular users, and for the ip address you want to ignore login requirements for, you set up an acl and add that ip adress to an ip_log file that the acl refers to. You then grant full access based on that acl using http_access in squid.conf.
You can also set this up to work on a dynamic IP address.