Can a Solenoid be driven from one of the pins of a Servo controller?
Go for the simple relay control. Try this one from the same site you linked to:
http://www.robotshop.com/en/devantech-2-channel-usb-relay.html

The servo driver ouputs a pulse train to control the servo position. Seems like an overcomplicated way of doing things by modifying it as an on/off controller.