My thought process follows.
Just thinking about the main issue of journal-based MMORPGs, I think the actual answer might be something along the lines of creating a new server running the LJ code, with some tweaks to make it work correctly. LJ wasn't strictly designed for RP, and the vast majority of users are in fact single users.
Either that, or a client that supports both a) managing multiple logins/fast switching, and b) will work to make changes to LJ settings (navbar/ads/etc.)
Actually, a client sounds like the best idea. I have my head half-in design space right now, because of dealing with bloody spreadsheets at work, and that sounds like it would be the ideal sort of tool to deal
with the problem of managing vast multiple journals where you'd really like to have a list of all your RP journals, the features that can be managed laid out in columns, and a "select all" tickybox to apply the same change to all the journals at once.
It would of course require updating the client to keep up with the new features added to LJ, but the main problem seems to be "do the same change to umpteen billion journals after I have looked at it and decided I do not care for this thanks", the problems involved in the vast scope of the management of all the journals.
(Now, if only someone with the LJ code background to back this up could tell me if that sounds like it would work...)