[Exim] Director that only issues "headers_add"

I'm trying to write a director under Exim 3.35 (Debian) that does nothing
but adds a header. I've tried several things to accomplish this but they
all produced undesirable side-effects (new_address=, driver=pipe, etc.).

Is there something like a "driver = noop" available? Perhaps a transport
that I can call to has no effect if provided the right options? Is there a
another/better way of accomplishing this?

Any advice appreciated. Thanks. - John R. S.