On the client I can use
Meteor.absoluteUrl() given that your
ROOT_URL env variable is set correctly.
See the following docs: http://docs.meteor.com/#meteor_absoluteurl.
Meteor doesn't know the outside-facing address of the proxy that it's sitting behind, and the (virtual) domain that this proxy was accessed by would have to be forwarded to the Meteor app for it to do what you are asking for. I don't think this is currently supported.