I like to boil cut-up potatos in chicken stock when mashed potatos are on le menu. To my chagrin, my wife kids do not care for the additional flavor. Same goes with elbow noodles boiled in chicken stock.
Pre-boiling baked potatos is a new one on me. How long to boil them would, to me, depend on how you like the skin's texture. I eat skin & all. Most people appear to want a skin sturdy enough to dig the meat out of.
If I were to 'speriment with your question, I would probably cook the potatos in stock for 15 minutes at a raging boil. Remove, drain, dry, season, and then plan 45-60 minutes in a 350 degree oven.