On a related note, there is an unsubstantiated rumor that if you aren't signed up, you'll go red if you kill someone unflagged on the field. Is this totally made up, or is there some truth to it?
Also, if you kill someone that's at war with you on the siege field, do you still lose 1/4 even if you aren't signed up? If so this change is meaningless. Siege jumpers just need to war whoever shows up at the siege.