Links to hard information about countries with high rates of greywater recycling would be very helpful in influencing areas that have unfavorable laws.
It's very difficult to find any hard numbers. Countries that currently have the highest greywater usage are most likely countries with very poor water-supply systems (think Congo, Liberia, Palestine). These are also the countries that do not have the resources to keep track of things like this.
Alternatively, to get some indication of interest in greywater reuse you can look at Google Trends. It shows that most searches on grey water/gray water/greywater/graywater/grauwasser/eaux grises are currently done in:
Australia 79.19 USA 40.20 South Africa 26.01 Canada 25.80 New Zealand 18.36 UK 16.83 India 7.07 Ireland 6.12 Germany 1.51 France 1.00
Note that the numbers don't mean anything. They are just a particular amount of interest relative to each other. I've calculated them by looking at the average and regional interest for each keyword. Also, this data is very much biased towards English, German and French speaking countries where many people have internet.
BTW, if you're interested in a document that describes the policies of certain countries, then you may find this PDF interesting.