Encouraging people to make better OPs has already been difficult, but if they feel there's a better process than bugging you in PMs about replacing them every time they need editing would help a lot.
I can imagine a thread having two new functionalities to make the monster threads less awful - the first is a form or something to submit OPs that possibly has a collaborative element (the Chantal thread has used a Google Doc to collaborate, although it's not done yet). This could be like your old idea of integrating the wiki for the first article of big threads and I think would be the easiest way. I think the wiki being on a separate site and not embedded into the forum makes it seem 'remote' and gets less eyeballs (which is true), making it less useful.
The other very important thing the big threads need to make them less of a nightmare is a separate box at the top of the first post that is essentially a contents page separate from the OP. The OP can have its own chronological biography in as much detail as the followers of the cow want to provide, but the contents page could simply be a series of titled links to milestone updates in the thread - eg, each new update for Onision's legal problems, or each new fad diet with Chantal. I'd imagine it as something like this (sorry for MS Paint):
Also a way for people to propose polls within the thread without having to have an active OP would be great, perhaps a voting system of "good poll/shit poll" or whatever.
The potential html/browser lag impact of the contents also need to be resilient enough to withstand the threads that are active enough to want to do a weekly "what's happened this week" type update, so possible the "see more" could go to a new contents page once it reaches a certain amount of characters, instead of just expanding a collapsed section.
If too annoying to code a solution to make a collaborative OP, perhaps have a separate discussion page for improving the OP, like wikis do, to prevent derailing the thread. Sort of an easy to view background layer to the thread for people who want to maintain it, but can be safely ignored by those who don't.