> Note I said "at least the same byte order".  :-)  Yah, cd9660 does this, 
> but I've gathered from a discussion or two that it's non-trivial to do 
> this with FFS, at least.

Why is it "non-trivial"?  The FFS has a magic number in the superblock,
allowing one to sense the byte order, which is the only real fundamental
problem in this kind of thing.  Granted, it's a pain in the butt to go
through the FS code and put a byte-order-correction layer in to all metadata
reads and writes, but still straightforward.  (Note that I'm using "trivial"
in a sort of mathematical sense, i.e. "a solution exists"; you may be using
"non-trivial" in the "way too much work" sense, with which I'd at least
partially concur.)