A FetchTV subscription to the Vibe or Ultimate pack comes with free access to WatchESPN. Would watching content on WatchESPN be covered by unmetered mobile data if MobileTV Streaming is activated on your Optus account?
If its a Fetch Channel then yes. You can always check by checking data usage when you wake up in the morning. Set the TV running for 30 minutes. Then don't use your phone till midday. The total usage should still read about the same (although if you look at the daily usage break down that will show data used (but uncounted)