You could probably make a script with curl that will login to smartsheet every 5 minutes or whatever, downloads the html5 dashboard page and save it somewhere. Then your media player can access the saved data.
As @Romo said, login details are not sent in the URL.
Yeah. I was thinking it was a long shot. I have been trying to figure out ways to create a local dashboard or similar to what you were saying that does that, or pulls info via an API and then the signage player connects to that, sans login.