I did this, and pressing ESC at the right time I was able to join Lightbringer and make a character.
However, Lightbringer is a normal public server, so this achieves nothing. When you try to do it with a server that isn't public (ie. Northrend, Deathforge) it just sends you immediately to the realm select screen, so it doesnt work.
EDIT: Holy crap, I just figured out a way to get into the Arena Tournament Realms. I don't have a tournament-flagged account, and I can now select both tournament realms and get into the character select screen. Only problem is, I have no toon in there, and can't create one (character creation disabled on these realms?) I can reproduce this at will, but only with the two tournament servers so far, no other locked servers.
I'd be willing to explain my method, but only if you can give me access to a private forum or something.