My understanding is that SNY-HD is not a 24/7 HD feed on E*, and this is probably why. If FSN Bay Area (Giants) and FSN Florida (Marlins?!) can do away games in HD, surely SNY, with its good ratings, can fork out the $$$.
SNY-HD / Mets away games are not available anywhere. Even YES does not broadcast Yankee away games in HD even though they said they would before the season began. I know this because I have TWC for SNY-HD and I also get YES-HD on that 24/7.
Are you sure about the FSN stations you mention? I see you are in Brooklyn so can somebody else chime in on the Giants and Marlins games?