I was going to suggest something in my previous message but apparently I
forgot.

> > > * be able to have the code itself easily (and
> > >   conditionally) inhibit them - as a whole or by type
> >
> > Like I said, with-no-warnings inhibits them locally, but I perfectly
> > agree there should be a way to suppress warnings on a file-local (and
> > maybe dir-local) basis.

Have you tried setting byte-compile-warnings as a file-local-variable?