
I don't think you can make that length of lead that will safely hold a bigger dog for training/walking outdoors in a multiway/police style, firstly because you would need to put in joins which would need to be really thin and delicately done or they would ruin the look of the braiding, and secondly because each point where you would put a ring to adjust the length of the lead, you would either have to convert back to a flat style, or work the ring onto some of the strands of the braid and then any pressure on them would pull the braid out of shape and risk breaking.
It certainly isn't something I would consider making, because I cannot envisage a way of making it safe (and I have been working with leather for nearly ten years now).