I hadn't considered this aspect of electronic pitch calling. While I have not researched these systems extensively, security would certainly be important. But compare and contrast a pitch calling system with a banking or investment on-line system. These systems manage billions of dollars and handle many thousands of public transactions per day. My assumption is that it would not be difficult to protect a system that has only 30 locations and does not provide public access to any features. Moreover, TV will still be using strike zone approximations during broadcasts, both to enable viewers to visualize the zone and to enable their graphics people to evaluate and fine tune their graphics. Any tampering with electronic pitch calling would be very easily noticed. So even if a system is hacked, I think it would be nigh on impossible to use what would have to be very subtle adjustments in the strike zone to significantly affect the outcome of an individual game or a 7-game postseason series, or for that matter even the regular season. There is just too much else going on in a baseball game.
So then people will say, well, if hacking is so unlikely to influence an outcome, why have the system in the first place? To me the response is obvious. Human umpires are much more prone to outside influence and much, much more prone to error than a properly-protected electronic system. Remember, it's all about getting the calls right so that players are the ones who determine the outcome of a game, not umpires or anyone else.