Depends on how it is coded. If it is done correctly you'd only need code to check for authentication, and you could have the code in another file and just require() it.

Also, you should be able to use .htaccess files by editing them on another machine and then uploading them to the server. Your only problem would be creating a htpasswd file, which you could overcome by using htpasswd on another machine. Don't quote me on that, as I've never tried it, but it should work.