> If we do the second, the opaqueness
> argument does not apply anymore,

i think it still does.
consider that you write a new filesystem and want to make its filehandles
endian-independent so that you can move a filesystem to another nfs server
easily.  (maybe with a user-specified fsid.)
with the current fid_len usage, you likely need to invent another hack.

YAMAMOTO Takashi