Query variables and URL rewriting magic

So, I’ve searched and searched and I’m most of the way to where I want to be with this code…

What I’m trying to do is create a site that is targeting one of four different demographic types — students, parents, teachers and counsellors. Each one has a different presentation of the page(s) they are looking at — mostly different sidebars and possibly some different text on the page.

What I’ve got is a query variable that I’m adding to the url for each, so there’s a link on the home page for students to go to the /my-page/?dem=student version of that page and parents to go to the /my-page/?dem=parent version of the page. That’s all working.

Now I want to make /my-page/?dem=student turn into /student/my-page/. Which I’ve done using query_vars and rewrite_rules_array filters. Bingo. Now I can go to /student/my-page/ and I get my custom content that’s just for students.

What’s the problem?

So, the idea with this site is it’s a bunch of colleges and they’re presenting different information for the different groups and there’s like 30 or so different campuses. So instead of /my-page/ it’s really going to be like /campus23/ (or, to be more accurate /cityname/). Each campus is a custom post type post.

My problem is that everywhere I look at tutorials and examples of rewrite_rules_array filters, it’s usually something like this:

add_filter( 'rewrite_rules_array', 'my_new_rewrite_rules' );
function my_new_rewrite_rules( $rules ) {
     $new_rules = array(
             'students/([^/]+)/?$' => 'index.php?pagename=my-page&dem=students',
             'parents/([^/]+)/?$' => 'index.php?pagename=my-page&dem=parents',
             'counselors/([^/]+)/?$' => 'index.php?pagename=my-page&dem=counsellors',
             'teachers/([^/]+)/?$' => 'index.php?pagename=my-page&dem=teachers',
     );
     $rules = $new_rules + $rules;
     return $rules;
}

…and my problem is that I don’t want it to be just pagename=my-page, I want the rewrite rules to apply to any post or page that has the ?dem={querystring} variable attached to it. Because otherwise, that would mean creating a list of rewrite rules that applied to every single campus as well as any other pages that may be available to those groups.

There has to be a way to do this, right?