If the main object you're working with is using its array 0 for some other vitally important purpose so you can't just fill it up with miscellaneous neighbor IDs for a phone book dialog, is there a harmless dummy object that I can create just to store the phone book dialog array? Can I put all the neighbor IDs into a sim's array 0, or does that break things? Do sims have array space? I'm actually not sure. I don't want to have to do something stupid like create a contentless object just to hold an array.

The game does have an item called 'ArrayHolder' GUID 0x935DC2EC, it is used by vehicles to temporarily hold the array of sims travelling somewhere until the travel controller is ready. It won't self delete so if you use it you'll need to keep track of and delete it when done. Sims do have lots of arrays, but they are used so I wouldn't mess with them.
Why not just use array 1? You can add an extra array to an item in it's OBJD in the Data Space section.

Thanks, that's helpful. I'm guessing the ArrayHolder object was present in the base game and doesn't have any EP requirements?

As far as I can tell, the phone book dialog requires you to use array 0, there's no way to tell it to use a different array.