jtesp, take a look at www.ayoungkim.com
The entire site is done in exactly the way you just described. Nearly all of the text content on that site is editable by the client via administrative web pages. In this particular application the text files are being written for use by Macromedia Generator, but the concept is the same using Flash MX or any other application that uses dynamic input, be it from a text file or a database.
mdipi has you headed in the right direction. The admin page would load the existing variables from the text file as the initial values in a form. The client could then modify those values as needed then submit the changes which would then be rewritten to the text file.
The admin page will need to be password protected for obvious security reasons.
