So what happened to good ol' ljsession? Well, if you log in via login.bml, it gets set to the value of a domain session cookie that ScrapBook can recognize. Problem is, the creation of a domain session cookie can only happen on LJ's servers, because they require a lookup/generation of a time-based randomized "secret" that's stored in LJ's database. Everything else that goes into a domain session cookie, I can construct, but without that secret, I can't actually create one, and LJ's client interface provides for no such thing.
I could, probably, theoretically, do something involving making requests to login.bml and scraping the results I got, but that would be a much more complex, probably error-prone, and ultimately disgusting procedure, so just so everyone knows, I'm considering "ScrapBook login compatibility" to be way the hell down towards the bottom of the feature pile. If someone wants to find someone at LJ to talk to to add a "generate a domain session cookie based on your existing master session stuff" client protocol call, that'd be awesome, but I really can't be arsed to do the necessary "which comm do I post to, and how can I be sure I talk to an Actual LJ Person and not a Volunteer Who Can Only Pass Things Along To Actual LJ People" investigation myself right now, when I have to worry about getting myself organized to work on the chunks of code I can readily improve myself.
Okay. End of tl;dr rambling and ranting.