If you want the setup for the subscribing sites to remain simple then I think that referrer is probably you're best bet.

If you can make it more complicated then the solution would be to get their server to fetch the page, or a token that allowed access to the page.

For example:

• Rather than linking to your pages directly get their server to proxy requests to your server - then you can track their proxies IP directly.
• Rather than redirect to your page directly, get a CGI on their server to request a URL from a CGI on your site (which will expire after an appropriate period). They can then redirect to this temporary URL and you can track requests to the url-giver CGI on your site from their server.