1l water = 1kg (not incl bottle, or plastic bag).
To assist with cramp I usually mix a level tsp of salt (or a less depending on how hot likely to be) into a 750ml water bottle.
As for the plastic bag/stream option - on the first half, we crossed a stream on the top on Sunday (after turning left at the stile just after the trig and where we crossed the fence using the stile). There was also a nice stream at the bottom of Emmi's secret descent.
Don't know what the second half provides, but certainly if you can make it to either one of those without needing a drink, then the plastic bag option could work.
Not too sure I'll be experimenting with this on the race though. Will see if I'm allowed out for this recce, I might give it a whirl then.