External URL for CIVIMail on localhost installation
This is only a partial answer, but the main challenge will be getting files to save to the remote server. Retrieving them will mainly involve adjustments on Administer > System Settings > Resource URLs.

If you can get a local folder to sync to a remote file host, that might work well.  Long run, a nice feature would be to have an option to save files directly to S3 or something.