I think an NPC journal is a fantastic idea. For as much as RIB ballyhooed about cutting down on guest stars, that never really happened. And they quite often play a big role with the drama of the week (like Cooper, or David Martinez, or the Messrs. Berry, etc).
Personally, I think the NPC journal should really just be for guest stars, and only for the week(s) they're on the show. So like, we can't NPC Roz each week cause if that happens, you might as well build her a journal and have her be one of your players, but someone could play her on weeks she's on the show.