Unusual Situation: Non-CI subfolder within CI - how to access it directly?

[eluser]behdesign[/eluser]
I have an unusual situation - I'm replacing the admin interface of a website with CI-based code. As I'm replacing the interface, I'm ajaxing in all of the CI forms, etc from CI. In order to get around cross-domain issues, I've decided to put the existing admin code and CI together in the same subdomain, with CI in a subfolder.

The problem is this: that the existing admin code uses a very crude cookie-based authentication, and any ajax request to get to CI is redirected by the existing code to a login screen.

So I thought perhaps I could flip the script: put CI in the root of the subdomain, and the admin code in an /admin subdirectory off the root. The question is: how do I alter the .htaccess (or CI) to let requests to the /admin subdirectory go through?