The easiest way I've found to do this is to use php to create the xml file that flash loads. Use some variables in your php file that filters the results to the specific client and put that in the link. For example, if you want to filter by client name you could have the filter simply be their name and the link would look like http://www.yoursite.com/page.php?name=clientName. This would filter the results that come into flash.
To give an overview of one such page I built for a client, I have the filter load with the php page that houses the flash file. Create a load variable inside the html that sends that variable into your flash file. Have flash send that variable when it loads the xml file (use a php extension with the ?name=clientName instead of xml so that it dynamically creates the xml). The php page builds the proper xml file for your client then sends that back to flash to output. It sounds long winded, but works like a charm for me.