[Exim] Passing message to run{}?

This may be another RTFM question, but I can't find all the pieces.

Want to try a Bayesian spam tool (spamprobe) that normally uses
procmail. It accepts an email message on stdin and puts the text of a
spam rating header on stdout. Seems like exim could handle this itself.

The exim specs suggest that the run{} command might be used as a
alternative for a pipe when you need to actually use the result, but I
can't see a way to pass the message as stdin to spamprobe using run{}.
And neither a pipe transport nor a transport filter seem to accomodate
this situation. Was hoping to be able to do something in a router like:

headers_add = X-Spamprobe: ${run{spamprobe train}{$value}{failed}}

Am I missing something obvious? My other option would be to write the
message to a temp file, and have spamprobe examine this. But I can't
find a simple way to do this either.

One recent post noted that getting tools for procmail to work with exim
should be easy! Can someone point me in the right direction?

Thanks,
Brad