You can use conditionals around to Keith's Mod-write hack to restrict its implementation to guest only... just check out my website... i have installed this hack succesfully (username:test, password: test).
Have a look at the Site Navigation Menu (left hand side)... as a guest, you would see some links with .html extensions but as soon as you login... the same links convert to .php extensions... and my google ranking are also improving dramatically after i implemented this modification...
i am planning on using conditionals around every modification suggested by Keith...
Regards
in that Kieth's thread on daniweb... the .htaccess file in the packaged in the hack thread is wrong... if you read through the 2-3 pages under that thread, there is another .htaccess file provided which worked perfectly for me.