Instead of null ids, you could give your unsaved entities unique ids that are negative. This would require a smart enough ValueEncoder that would NOT attempt to lookup negative ids from the database.
It's all sounding a bit hack so you might prefer to pass around a wrapper object which knows if the object has been persisted yet or not. Perhaps an entity that has been persisted might encode as "P:123" and a new entity not yet persisted would encode as "N:123".
