Login & Error Message Customization

We are working on a redesign for our e-commerce site whereby we now want to password protect all the category & product pages since the offers contained in these pages are available to members-only.

We understand that we can change the default error message for "category is no longer available" under Setup > Website > Customize to a redirect that sends the user to the member login form, however this solution will not work for our site. Let me explain further...

The problem is that we send out monthly e-newsletters to our members with links to our category pages. Once a member has joined the site their login information is stored in a cookie so they don't have to login again with each visit. So when they click through from one of our e-newsletters to a category page where an offer may have expired, we need them to get the default "category is no longer available" message. If they were shown the login form instead they would end up in a confusing loop and would have trouble navigating the site.