I would be most concerned about a clay pond. Most clays will bind nutrients quite easily and asymetrically, leading to a poor nutrient feed. I'd highly recommend lining the pond with a non-reactive layer. There are small valves that can be used to configure a slow drip from an upper reservoir to the plants; the ones meant for 1/8" or 1/4" drip lines work well. I use longish (maybe 8-12') lengths of the line instead to provide a small flow. The valves may clog, or be accidentally closed, while the lines tend to stay open and in service. The drip line is so cheap that the line also costs quite a bit than the valve. If you use a drip line as emergency backup, you could split it into multiple ends which could be placed directly on top of the pots. This way you do not need to have a large amount at the bottom, and the plants will be more likely to get at least a little water. Even in my pretty dry environment, a drip feed of 200-400 ml/day is enough to keep the plants alive in an emergency, so you don't need a huge emergency supply, in my opinion, especially if you were willing to hand supply some water to the system every few days in an extended power outage.
Renais
