Is it possible to use composite keys where one of the properties can be null?

do it !! and answer by yourself

It is possible, but I won't recommend it.

I've spend a lot of time trying to get it to work.
I wanted a composite key which contained two Foreign keys. I suspect this is possible, but not easy.

I worked around it by using a normal Primary Key, and the foreign keys in a unique property.

If you want to give it a try: http://docs.jboss.org/hibernate/core/3.3/reference/en/html/mapping.html#mapping-declaration-compositeid