You’ve got the right idea, and frankly, someone with more experience than myself might figure out how to do this with a “simple” dry-contact Z-Wave sensor or hacking a ZigBee or Z-Wave Contact Sensor, etc..
But if I were to do this without further research and for a little extra education, I would recommend searching the Forum for ESP8266 … this is a mini-Arduino (programmable MCU: Micro Controller Unit), which can easily read the input from a $3.00 photo sensor and, the ESP8266 has WiFi built in, which means it will connect to your LAN and then can talk to SmartThings.
This second method is less complicated than it sounds and opens you to a world of possibilities … like wiring in a second light sensor to your Dryer from the same ESP8266, for example; and other project ideas around your home.
Here’s a an example of what the ESP8266 can do:
Of course, the SmartThings “ThingShield” (ZigBee based) was a slightly simpler and slightly more expensive option; but they discontinued this. I use mine to control my Projector, Projector Modes, and Screen.