Inspired by Wei_Dai's comment, and assuming that SI should have a single logical document, kept up to date, maintaining its current case:

SI could try to get their "previously unpublished research (not published even in blog posts or comments)" out there in the form of roughly written and loosely argued blog posts, before worrying about creating this document.

Pros:

• As Wei_Dai observes, it's easy to spend lots of time laying the groundwork and never getting to the actual building stage.
• At least some people not in SI are at least roughly familiar with the groundwork already; those people would be better informed for these blog posts and might be able to usefully contribute to the discussion.
• The posts would provide material later to be made part of the single evolving case document.

Cons:

• People will try to use them to evaluate SI's case anyway, despite the disclaimer at the top.
• If I'm more convinced of SI's case from reading it, great! If I'm less convinced, hold off, we haven't finished writing it up yet. Are we asking people to suspend Conservation of Expected Evidence? (I don't think this is a problem, but not confident enough to leave it out)
• Now we're converting material twice: once from blog post to evolving document, and once from document to papers.

My conclusion: I like this idea a lot less having written it up. However, I still like the idea of getting it out there first and polishing it later. I'd rather say, start work on the evolving case document immediately, but write the new stuff first, and worry about the stuff that's already out there in other forms later. Or pages on old stuff could start life simply linking to existing sources, with very little new text. No matter what disclaimers you put on a blog post, people will treat it as a finished thing to some extent, and major edits during discussion are problematic. Linking to a revision-controlled, evolving document like a wiki doesn't have the same feel at all.