How feasible is something like this? I think we'd have a hard time telling a country to tear down farms and other developed land to let it return to rain forest.
There are increasingly Tax breaks for necessary investment available in cities in the US doing this (1)and certainly many other countries/regions e.g (2) have various on going policy initiatives for similar developments.
all in all - its highly dependent on the building and local planning regulations - but more and more places are open to supporting it and hopefully actual adoption and implementation accelerates soon.
(1) http://myplantconnection.com/green-roofs-legislation.php
(2) http://ec.europa.eu/environment/nature/ecosystems/background...