Can you hang the Ortlieb so that the roll top remains open while filling the sack? Otherwise I think it would be hard to keep upright while filling.
The snap at the top can be clipped together and allows you to use a grab handle when collecting water such as in the video for the Katadyn Base Camp gravity filter. In fact the bag supplied looks identical.
http://www.youtube.com/watch?v=w_0aPpTKD24&phttp://www.youtube.com/watch?v=O_7N_xaURvoThe Katadyn base camp uses the cheaper filter cartridge from the Katadyn Hiker model. The Katadyn Siphon filter is also compatible with the Ortlieb Water Sack.
http://www.sailgb.com/p/Katadyn-Siphon/I may get one of these later. Using this combination allows up to 20,000 litres of water to be filtered using a ceramic cartridge rather than to 1000 litres using the Hiker cartridge.
The bag also makes for a neat compression sack for storage of some cold weather MoD issue softie bivi jacket and trousers.
You may find the MSR dromedary bags slightly more robust for carrying in a back pack though and they can apparently be used with boiling water.
My main use for the Ortlieb bag was really as a totally waterproof compression sack and solar shower combo. Water carry and gravity feed filter were really secondary requirements. Carrying 22lb of water whilst hiking in Scotland doesn't really make for great deal of sense.