lucy24 - 9:37 am on Jun 27, 2013 (gmt 0)
You've got the redirect down solid. But where's the rewrite?
Once the browser has made its fresh request for "Master-Product-Key" you have to tell the server to go back to long-boring-url.php?more-stuff-here to find the content to make the page.
But you can't write the rule until you've got a firm grip on what's really happening.
Crystal ball, now working overtime, strongly suggests you've misunderstood what a rewrite is. You would not be the first person to get it backward :) (It is not possible to have the "rewrite" concept a little bit wrong. It's either exactly right or 180 degrees off.)