[exim] Availability of return-path to Exiscan

I see that it would be very useful to send a return-path header through
Exiscan to Spamassassin. Currently, that header is not passed. I even
have this in the acl just before SA is called in Exiscan:

warn message = Return-Path: $sender_address

But apparently it is not sent because SA does not see it. Is there any
way to have this header sent?

Thanks,
Craig Jackson