MLB Extra Innings HD Beta is available to Xfinity customers on 03/29/18. For details about the Beta, eligibility, and how to access that content, please see here: http://forums.xfinity.com/t5/TV/ANSWERED-What-is-the-Xfinity-MLB-Extra-Innings-Out-of-Market/ta-p/29...
It took awhile but I have finally figured out how to access the beta for mlb and watch the games in hi def. I spoke with 14 different reps from Comcast and not only didn't they know how to do it, but none of them even knew what I meant when asking about beta.
For all non-Red Sox fans here's how to do it. The beta option is no longer listed in the channel guide as it used to be, even though it still is for NBA and NHL.
Go to channel 1 (Xfinity on demand Channel) and hit select.
Scroll to right and select Sports
Under Featured Sports scroll right and choose MLB (picture of Bryce Harper)
Scroll down to Featured and choose MLB Extra Innings HD (Astros player sliding home)
BINGO! Select your team and begin watching.
Hope this helps. Go Yankees!