Just out of curiosity, rather than making it extremely difficult, do you have to add to the end of the url?
If you could do something like
http://site.com/identifier/page.php -- you could use a mod_rewrite to strip out the identifier. A simple method to do so would be to use something like an old webserver used to use called prestates -- http://site.com/(blah)/page.php, the mod_rewrite would just strip out the (blah) portion and ignore it. Your html would need to be made so that it was all relative, i.e. no <a href="/site.html"> but rather <a href="site.html">
If that would work, it wouldn't require any modifications to your files on the fly and would be very quick.