Only time I ever saw a zone lockout was in XI when Beseiged was first introduced. I think Al Zhabi was able to hold 600(?) players at once before it would start to lock out people. At first, if you were the unlucky sap that logged in Al Zhabi and tried to log in during a beseiged, the game would bump you back to the character selection screen.

SE promptly fixed that by tossing all players trying to enter Al Zhabi into Whitegate, and if you kept trying to zone to Al Zhabi, you either got lucky and got in or you'd just get bumped right back into Whitegate.

I can see SE doing something similar with XIV. Though I can't think of any reason any zone would get THAT full.