[Exim] Rewrite Header

Hi

I am searching for a way to rewrite subject header.
I cannot use System Filters since the rewriteing condition depends on
information of the routers.
And as far as I know System Filters are executed before routers are
processed...
And as I have no system users [but only virtual ones], user filters seem to
be impossible.

Is there a way to do it? Preferably inside a router?

Regards,
Steffen