fallback programmable completion for many or all commands

[Britton Kerin]

I like the programmable completion but I'm having some trouble getting it
do do exactly what I want:

I have a directory called fsbmk with links into my tree structure.  I'd like
all commands to fall back to looking here if they don't find anything else,
or at least to merge all completion from this source with others.

Problems:

1. Using complete -C whatever for every command in $PATH is too slow
    to do at startup.  But I don't see another way to add compspecs for many
    or all commands.

2.  I would like to operate without disturbing existing, potentially more
    specialized completion.  The -D option doesn't work because it only
    applies when no compspec exists (I have some big library of compspecs
    from somewhere, though sorry I couldn't remember what).  One can
    of course use parse the output of complete -p and make a wrapper
    function, maybe this is the best and intended mechanism.  But its a
    little cumbersome.

Thoughts?

Britton