I doubt that it matters where the 1 liter of liquid is (except in your body) as long as you have it when exiting a checkpoint. They probably only stipulate rucksack as most people tend to use them for all kit and liquid. As has already been said you can dump some as soon as you exit a checkpoint.
Would 1/2 liter not be a bit heavy in each hand though? Like me, no sticks for you then!