Interesting about putting  plan9 in the middle; that feels like a wart,
and it's hard to understand how extra protocol translation can make
things that didn't have a way to be done work.

I too am really unclear on the "extension" notion.  If this is a set of
extensions to plan9 that are reasonably portable (say, to all posix
systems) and implemented in portable FUSE/plan9 code, then that sounds
fine.  But doing anything that only works on Linux sounds like a step
backwards.

Can you point to the plan 9 fuse client you recommend?

Can you explain how authentication works?  Is there crypto in the p9
protocol?  How are other plan 9 clients on the local machine denied
access?

Presumably the coda p9 server is localhost only.  Is that right?