I think you got me wrong here. What is confusing is the fact that if you are aware of the PacketWriter you might think that this thread will call the listeners, which is wrong.
The next problem applies to developers who are not aware of that additional PacketWriter thread. The method is called #addPacketSendingListener but the Javadoc talks about “writer thread”. The developer may
has no clue about what this Javadoc is trying to explain.
Personally I think you just can adapt the Javadoc of the #addPacketWriterInterceptor method with a remark that the listener will go to see a possible changed Packet (modified through the Interceptors).
Btw, the Javadoc should state that #addPacketSendingListener listeners should not try to mutate those packets if the Packet#toXML method is not threadsafe. (even if the method is threadsafe they must not mutate them at all as they would just act as another Interceptor with a very bad concurrency experience )
As you already mentioned that all this will be changed anyways with XEP-198 you should just keep this thread topic as a reminder to avoid the problems again