I see no opportunity for code sharing here that isn't already being
   exploited.  The only difference is how the special-case hooks get
   tripped; in one case it's via a very special device driver, in the
   other it's via the filesystem-specific open routine.

so, why don't we just remove them from fdesc and have fdesc mount
directly on /dev/fd ?

sounds like it would only remove some function calls in the case
of fdesc being mounted:  speeding up things, really.  and it would
fix the outstanding pr's on compress/fdesc (maybe as many as 5)  :)