Question: Is it possible to use one of the USB-Serial device GPIOs (CY7C65211, CY7C65213, CY7C65215) as an edge-triggered interrupt?
No. It is not possible to use a GPIO pin as an edge-triggered interrupt input pin for USB-Serial parts (CY7C65211, CY7C65213, CY7C65215). There is no option to configure the GPIO pin as an edge-triggered interrupt using the configuration utility. Also, there is no API to detect the edge of the input GPIO waveform.
You can only poll the GPIO using the host application. There is an API [CyGetGpioValue ()] to get the current state of the GPIO. This API can be called from the host application.