At the risk of being ostracized or yelled at (though it seems I'm always willing to risk those), it looks like someone needs to get this discussion started, so I'll make three statements on the issues you raise in your question...
1. The definition of "water footprint" is still ambiguous - governments, companies, NGOs and research will all have different ideas on what it should or should not include.
2. While there are international conventions and treaties, bilateral agreeements, and national policies, there is no such thing as "global policy" and no international organization empowered with the declaration of policy. One might suggest the UN and its member agencies, the International Criminal Court, the Geneva Conventions, etc. but none of them set "policy" that all parties/members/signatories must adhere to.
3. With 260 international river basins, 190+ countries, and a global trade web the likes of which has never before existed in history, there are still only a few multi-lateral agreements over sharing of surface waters, and even then only by geographic neighbors. Groundwater is not cooperatively regulated, not even between neighboring states in the US let alone between countries. Virtual water is still a mystery to the very companies that trade the most of it around the globe, let alone the governments who might regulate that trade. Water remains a grossly ignored and ill-understood resource, not yet a commodity, but always a necessity.
That should get something started here, yes?