The jack labeled IR could be an IR blaster for controlling a recording device like a VCR. Since cable boxes and even their software vary by location, you will need to provide more information to determine if that jack is a an input.
If it isn't an input, then control is just as easy. Both Parts Express and Smart Home have IR relay devices that convert the IR signal to electrical, send it over a wire and convert it back to IR. There are also wireless models. I've picked up some older JVC wireless IR relay thingies (technical term) for $25 off of Ebay.
What I worry about in your plan is putting the cable box in the attic. In the summer, my attic is HOT. 130 to 140 degrees hot. A cable box wouldn't last an hour in that heat.
