Unfortunately I think you'll have to familiarize yourself more with the existent decision theory literature here on LW or on the decision theory mailing list in order to understand what I'm getting at. I'm already rather familiar with the standard arguments for FAI. If you're already a member of the decision theory list then the most relevant thing to read would be Nesov's talking about decision processes splitting off into coordinated subagents upon making observations. That at least hints in the right direction.
(I have no idea what Will is talking about; I don't even see which things I wrote on the list he is referring to.)
Edit: Both issues now resolved, with Paul clarifying Will's point and Will explicitly linking to the decision theory list post.
Here's the new thread for posting quotes, with the usual rules: