I was trying, without success, to map a database scenario which would be one-to-one, but that the child rows for the parent may be missing from the database.

This changes the nature of the relationship I think, but I'm not really sure how to deal with it using NHibernate. Any thoughts?

You can map it as a many-to-one with not-found="ignore" and unique="true". Have a look at the end of chapter 5.1.11 in the reference documentation.

I'll look into that. Thanks for the suggestion!