personally i dont see where the torsion bar has anything to do with a roof load, the spring will never have more tension (load) than in its stored or fully open state, if you want proof unlatch the top and watch it pop up several inches. add the bars ect that you plan to leave mounted on the camper, place in the areas where they will be mounted, if the top is still held up by the spring your golden and not exceeding the spring rate.
thats my bit on why i think its perfectly fine to mount something on the lid, martyn says the warranty will be void, i'll take that to the bank, the design is poor and they do break. if you plan to get it fixed under warranty dont mount anything to the roof, i'm not worried about it cause i dont have a warranty, you will need to decide that for yourself.
i will be carrying a kayak on the roof to avoid a trailer. i'm not worried about the weight of the kayak while the camper is closed. and whatever i decide for mounting it i wont be worried about that while opening and closing the top.