I am wondering how to tell NHibernate to resolve dependencies on my POCO domain objects.
I figured out that methods like CalculateOrderTax should be in the Domain ob
As no-one seems to be able to answer your question at the moment I thought I'd suggest restructuring your code to remove the need for the Order to calculate it's own tax.
You could delegate it to a OrderTaxService which takes an Order object and returns an OrderValue object or something along those lines.
This will keep the logic in your domain but remove the need to attach it to your Order objects.