"The problem is, that status changes take place in an external system"
That is your basic problem. Essentially you have two or more independent systems, which is not good. Something occurs to me that you might be better off looking at an LDAP database for this type of problem. If you wish to use an external system, maybe use a separate-from-everything-else rule engine. Feed data into it from all sources which determine status and use the rule engine to make the decision, but first you need to get all the rule data into a single database. LDAP is difficult, but I can almost guarantee it is much easier than XML and xpath, and designed to be fast at this kind of task.
