Minecraft – Help with /scoreboard objective score abuse!

Create scoreboard objective:

/scoreboard objectives add inArena dummy

Run on clock to give people in the arena an inArena score of 1:

/scoreboard players set @a inArena 0
/scoreboard players set @a[x=<X>, y=<y>, z=<z>, dx=<X length>, dy=<Y length>, dz=<Z length>] inArena 1

To display the title to people with inArena of 1:

/title @a[score_inArena_min = 1] <Title>

The problem is that you can't target someone without a score using a score selector. So the solution, not surprisingly, is to give everyone a score. You don't have to change anyone's score value either; instead, just "increment" everyone's score by 0, in your case, with the following command:

/scoreboard players add @a hasJoined 0

Better yet, throw that command in a command block that is triggered off a 20Hz clock, and you're guaranteed that everyone always has a score.

The way this works is that everyone that already has a score in the hasJoined objective won't have their score changed, but everyone else will have their hasJoined score set to 0.