What a great project.
I assume you've seen this video,
I'd check out Darren Perks Hatbox Ghost mask
at Dynamic Design International
I assume you'll only project animated eyes on the mask and project the entire head (with animated eyes) in the hatbox.
You'll want to make sure you can get a single projector to focus on the mask and the hatbox given they're in different planes (i.e., the hatbox is at arms-length from the mask). I suspect two projectors will be easier.
Do you plan to (mechanically) animate the ghost's body (i.e., will the mask and hatbox be moving)?