Give the web adapter a configuration option to connect only to a single folder of published services on ArcGIS Server.
That way it makes it easy to manage services intended for ceratin internal and external routing. For example, if you have certain services open to the external public you can publish those to a "public" folder. Then have the web adapter in the DMZ acting as a reverse proxy only connected to the public folder. A web adapter on the ArcGIS Server inside the organization can connect to the root of the services to have access to both the public folder services and other services that are for internal use only.
