Since Externalized Properties works with interfaces, it makes it easy to integrate with dependency injection (DI) frameworks. it's as simple as building ExternalizedProperties, initializing a dynamic proxy from an interface, and registering the proxy interface to your chosen DI framework.
Another side-effect of being dependency injection friendly is that it also makes it easy to mock/stub out configurations/properties on unit tests. It's as simple as creating a stub implementation of the proxy interface or using mocking frameworks to mock the proxy interface.