While the current new bike with an IGH is usually using a full length casing I suspect this is to try to keep the cable/casing from becoming contaminated, the fewer ports of entry the less grit/water will get in. However sectioned lengths of casing work well too, just as has been the case with IGHs for decades. Do take some acre about the casing's bends, smooth large radius curve will have less friction than tight bends will have. How the hub's casing stop clip is aligned and where along the stay the last casing stop is the usual spot of greatest friction and/or contamination possibility. I suggest using casing stops that are slotted so casing removal for cleaning and lubing the inner cable is easily done.
The inner cable is typical der (1.2mm diameter) with the common Shimano head. The casing doesn't need to be gear/SIS stuff. Any quality lines casing can do. Just be sure to square off the ends after cutting to reduce the casing wiggle within the stops.
Not asked about but... I set up my IGH bikes so the flat land riding gear is one of the higher ratio ones (on my old SA AW that means 3rd gear). One can always coast down the steep hills if you can't spin well, but up hill and there's no cheating
Andy