Can Squid Intercept Certain Pages and Display a Warning Page Before Redirecting

[Steven Weintraut <sweintraut@xxxxxxxxxxxxxxxx>]

Hi

We have a group of users who are being very excessive with their use  
of personal emails on google mail, yahoo mail etc, despite repeated  
reminders to limit the amount of time they spend doing that during  
the day

Rather than take the severe route of just blocking those sites, I was  
trying to think of a way to use Squid as a proxy and when they visit  
the login for one of those email sites that it would first display a  
reminder page reminding them to limit the amount of time they spend  
doing personal stuff during work hours, and then allow them to  
continue on as they normally would either via a timed redirect and/or  
a hyperlink they could click on

Once the warning was display we would want to turn off the warning so  
that it didn't come back up everytime they went to read a message or  
click on some other url during the same session

I'm at a loss how to do this, but I would imaging someone has done  
something like this before, even it it's something we need to  
purchase versus download?

any ideas? thanks!