[Exim] Rewriting headers with transport_filter / contents of…

I'm using a transport which in essence looks roughly like this:

    local_delivery:
      driver = pipe
      transport_filter = /bin/modify-subject
      command = /bin/save-to-disk
      environment = <; SUBJECT=$h_subject:

(Never mind the obvious security hole.)

My problem is that $h_subject: is still the old subject as it was
before the transport_filter was run. Is it possible to make Exim
regenerate its $header_* variables based on the output from the
transport_filter before setting the environment?

--
Tore Anderson