Yeah, limiting it to one-off guest stars for a week at a time would probably be a good way to keep it under control, and make sure the characters who could more easily be fully fleshed out characters don't get monopolized by the NPC journal. There's some guest stars who are recurring that could merit full character status -- Jesse St. James, for example -- but in cases like Cooper, who lives in LA and really can't justify becoming a full character, it'd be a new source of interaction.
I'll see what I can do to set up a journal and hammer out some guidelines for using it tomorrow -- feel free to keep discussing, though, these are good points!