These photos show how I have fitted a 'Platypus' hydration bag (the 1 litre version) into the front frame triangle of my Moulton NS. The bag is held in place by a couple of velcro straps stretched between the frame tubes. The drinking tube is just long enough to reach my mouth. It is secured to the stem by the clip that comes with it and is wound around a handlebar map holder when not in use.
The same arrangement ought to work on any space-frame Moulton.
If you look closely you can see that I have found yet another way to mount a Cateye computer on a Moulton. The Cateye mount is secured to the upper front suspension moving block by a couple of cable ties. The arrangement is stable and works well. It leaves the centre of the wishbone stem free for a Garmin Edge GPS (which I don't use much).