I agree that using FUSE for coda would be good.

On NetBSD (5), coda mostly works, and I use it daily.  "pwd" is broken,
but I think that's it.

NetBSD has a BSD-licensed FUSE variant librefuse, built on top of puffs.
So far it seems that programs written to FUSE work fine on NetBSD with
librefuse.   http://www.netbsd.org/docs/puffs/