Well, technically, the slot doesn't have to be empty. You can read it to say it uses the slot, sure, but even if you cast the Anyspell itself from the Domain Slot, the spell you prepare is now placed into a spent spell slot.
So that means you cast Anyspell from any slot, and it'd go into your Domain Slot, RAW. I suppose that might mean you need one to cast it, but it's not stopping you from casting it multiple times a day.