after lots more googling I've partially answered my own question.
Seam 2 introduced ManagedEntityIdentityInterceptor which was intended to resolve the
detached entities problem:
ManagedEntityIdentityInterceptor has been disabled by default in Seam 2.1
JBSeam-2209 is marked as resolved for 2.1.1. Does this mean MEII is now safe to use in 2.1.1? Are there performance considerations? Is the problem with nested conversations resolved? The notes in JBSeam-2209 don't make it clear.
A container must not passivate a stateful session bean with an extended persistence context unless the following conditions are met: • All the entities in the persistence context are serializable. • The EntityManager is serializable.
The session bean class is not required to implement the SessionBean interface or the Serializable interface.