Not a redirect but a tunnel

Probably not the right group to post this but I'll start here.

What I would like to do is this:

Let's take a website that has many pages that each have been linked to by
many users. I don't want to redirect them to other pages but want them to
"tunnel" thru a new page on their way to the page that has been linked to.
This, of course, is without editing any of the linked-to pages.

This new page would be tunneled thru by anyone entering to any of the
linked-to pages but not everytime. Not as they continue along the indexed
pages but only the one time as they enter the site.

One option would be to redirect them all to a main entry point with a
..htaccess config but I don't want them to have to shuffle along to get to
where they wanted to be. Just to be "forced" to view the one new page,
click a link on it before they go about their business. Once they see it
they're done with it until they leave the site; seeing it again if they
return.

The site is pure html. No style sheets. No php usage or any thing else that
would've made life easier.

Any ideas?