I understand your project requirements and can have this completed for you today, no problem.
As you have correctly identified, this is not possible with cURL. I would propose developing a PHP class that creates a socket connection between the user and target page (passing their IP address in the process) but spoofing the header info to include a custom referrer, such as Facebook, Twitter, or whatever you wanted.
The only obvious downside, is that the user won't technically be 'redirected' to the target page, he will only have the target page's data returned to him.
However, I'm pretty confident that with PHP this the only way to accomplish anything like this.