I agree with PPs, it is not your duty as host to cater to Suzy's phobia unless it's a party hosted specifically for Suzy. Since it's an annual party, I would tell Suzy that as far as you know Curtis is the only one coming in a half mask, but you haven't really restricted it for anyone. Let her decide if she's comfortable coming or not.
Or, there's another option for her, if you're okay with it. Say everyone shows up and there's no masks at all. Then would you be able to shoot her a text and let her know that so she can decide at that point to show up? If it's a party that goes on for several hours and people are freely mingling, it won't matter too much if she's late, or if she leaves early because she gets spooked, right? Yes, it's above and beyond the standard host duties, but that way you're not setting policy on costume and you're still trying to include Suzy.
This year, I'm dressed as a character from Doctor Who. Human, in a dress and a jacket. Nothing on my head at all. But if I got a message from the host saying that nobody could wear face masks or prosthetics I'd probably be tempted not to go to that party. But that's just me personally, I can't speak on that for anyone else.