I have been playing recently with Squid and “Splash pages”, but this function works fine on Squid 3.2 and later; however Raspbian only offers Squid 3.1.
I looked for a ready to install 3.2+ version in the Webz but didn’t find one, so I backported the Debian Jessie version.
For the lazy ones, here are the links (try them at your own):
libecap2 (This dependency is not available in Wheezy)
Then install squid3 (install according to your needs)