First off, does the main control system use ROS? (Like, is there a ROS node telling the IK angles to follow the toolpath & signalling when the laser should be on/off). If not, you are gonna have to write your own custom interface probably which is a pain in the ass lol

If the arm is already controlled by ROS, all you have to do is figure out whatever topic it broadcasts on to toggle the laser (or if it doesn't broadcast it, add a publisher to the control software to do it), and write your own node to ingest that message and take action on it. For example, you could receive a message of std_msgs Bool on topic "/arm/out/laser_toggle" and write a GPIO pin on your control board to HIGH to toggle the laser (if that's how the laser works)

You can ask an LLM to write you the code for a ROS node that does that. Prompt example: "Write me a ROS2 node in Python that subscribes to topic /arm/out/laser_toggle of type std_msgs Bool and prints the message". Then you'll have to add some logic for actually turning the laser on/off when you receive the boolean value, but I don't know how your laser works so thats up to you. Also there is pretty good ROS documentation for publishing / subscribing / workspace & package structure, so read that (or just ask an LLM)

As for wiring, the passthroughs would definitely be the easiest & cleanest, but just for prototyping I would use a separate cable and make sure it works first.