From my understanding (and maybe a Seam team member can confirm this) all bijected attributes are disinjected (set to null) after method invocation. So any bijected attributes will not be included in the passivation of the session bean.
For any non-bijected attributes, you could either use transient or set them to null using @PrePassivate. The @PostActivate annotation can be used to retrieve them on activation. Hope that helps.
Should have specified previously, but for clarity the disinjection only applies to @In attributes.