Help – Editing Email Templates

I’ve got to add a variable, that’s stored in the customer (well, technically it’s in an EAV, but that points at the Customer), and I’d like to add that to my New Order Email. How do I do that? My biggest problem is, I don’t know what variables are available in the Email Templates. Sure, there’s the popup that should tell me what I’ve got to work with, but then I look at the template and I see a order_data Variable that I can’t see anywhere in the popup. So, any help on this is greatly appreciated.