I was doing some research in custom XML serializaton in .Net. The problem is the same as the O-R problem  for database persistence, the difference here is the level of support that .Net provides in helping with that serialization. I stumbled upon a great site for XML related information http://www.topxml.com and while going through some tutorials on that site came across this cool class called ISerializationSurrogate. You can read more about it here http://www.topxml.com/xmlserializer/surrogateselectors.asp

Basically it allows you to move the serialization code away from the object being serialized. A class that implements this interface can act as a surrogate for the class that it serializes. So that's another hook you can use to customize your serialization.