[Exim] Filtering: piping

Morning.

What is the simplest way to pipe the message through some process
while filtering?
I mean I'd like to pipe it and deal with the result like procmail
does.
Currently I see two ways -- using procmail or refeeding the process
output back to exim. Both are clamsy. Maybe internal some mechanisms
may be applied?

Thanks.

                   Ingvarr.