According to the API, you can leave the device serial number "unset" and it will attach to "any". In the documentation for Phidget, there's also a constant defined: ANY_SERIAL_NUMBER that says it will also allow you to attach to any serial number.
I've used this sort of feature on phidgets21 in the past. In the current phidgets22, when you leave that field unset or set it to the defined constant, then no device will be attached.
Products for USB Sensing and Control
