The latest version of CasaTunes now supports the Global Caché iTach Flex hardware and the Link Relay & Sensor Cable option. This hardware option adds four sensors (although only one can be used with CasaTunes) for detecting when a switch is closed (like a doorbell). Using this option, it is now real simple to integrate a common doorbell and have CasaTunes play a custom chime when the doorbell pressed.
The Simple Doorbell Integration Tech Note in the Support > Resources section of the CasaTunes website walks you through how to accomplish this.
Best of all, this solution works with all CasaTunes systems, including systems that include a CT-1 or CT-2.