Few questions

• The best way to handle this might be to use a database insert trigger to set the real serial number during save.  DevForce by default will re-query saved data and merge it back into cache.
• If you don't want to use a trigger, another option is to listen for the PersistenceManager.EntitySavingEvent, and in the handler set the real serial number on the entities passed.  This event is fired on the client before the actual save logic kicks in, and is not part of a transactional save.
• On the server, the only interception point currently available is in the Entity.SaveSecurityCheck, which when implemented is called for each entity to be saved.  You can set the real serial number here, but note that this method is called before the save transaction begins.
• Finally, there's really nothing in the IIdGenerator that knows whether primary keys are being used, so it's potentially doable to use a custom IdGenerator for this, although this would be an undocumented and unsupported use of the IdGenerator.  Id fixup (mapping of temporary to real ids) occurs on the server, before the save transaction begins

3) Our standard column binder for the combo box doesn't support adding new items, but we do have a sample which shows how to build a custom binder.  Email support@ideablade.com directly to have this sample sent to you.