Cooling and diluting over-reduced stock down with cold water I've just had a brown chicken stock on the simmer for the last 8 hours, not planning for that long but got called out. Nonetheless, the liquid has reduced by about 1/2 - 2/3. 

Would taking it off the heat now and adding cold water to both bring the liquid quantity back up and cool the stock for refrigerating be an appropriate rescue?
I have to ask, why would you want to add water? Chicken stock should simmer 8 hours. That is the optimal time to extract all flavours. What you have there is (probably) perfect. However, if you don't want as much flavour, add the water later, not now. Also, to cool, the best way to do it is in an ice bath or a cold water bath (one where there is a constant slow source of running cold water).