SRS and Sendmail aliases

A couple of the local users here are actually off-site so mail is
forwarded to them using the sendmail aliases mechanism. As we run SPF
I would also like to implement SRS for these forwarded users. Looking
at "http://spf.pobox.com/srs.html" an example shows the use of
 "|/usr/bin/srs ....."
in a .forward file and I believe that the syntax of the .forward and
aliases files are the same.

I have tried this but it does not work as sendmail attempts to pipe
the mail through stdin of the program specified in the aliases file,
and /usr/bin/srs does not expect this but simply generates the new
envelope-from header.

What is the best way of doing this?