[exim] content scanning with spamassassin in exim 4.50

I'm working on setting up my exim 4.50 system to do content scanning
with SpamAssassin through the data acl. I want to be able to do two things:

1) reject messages at a system threshold
2) add spam headers to the message, but using the recipients
spamassassin prefs (even if there are multiple recipients)

The first part is easy:

   deny  message = This message scored $spam_score spam points.
         spam = nobody:true
         condition = ${if >{$spam_score_int}{120}{1}{0}}

The second part is my problem. I basically understand the issue of
multiple recipients. I know that if I am considering user settings, and
a message comes in where two users have conflicting spam score settings,
I cannot both ACCEPT and REJECT a message at the data level. I
understand that some people get around this by enforcing single
recipients, etc. but this solution is not acceptable for my environment.
What I would like to know is whether I can still ACCEPT a message that
is at least less than the system threshhold, but at the same time, add
"Spam: Yes" or "Spam: No" headers to the message for each recipient,
depending on their own personal configuration of spamassassin in their
home directory. I'm just not sure if exim allows for this syntax in the
ACL condition, or whether a single message cannot have different
headings for different users...

If this is not possible, I still wonder how I might enable Exim to
"rescan" each message with "spamc" for each user so that I could at
least add Spam headers to messages that are not rejected. Users could
then choose what to do with messages that are less than the system
threshold -- do nothing, move them out of their inbox with procmail, etc.

Thanks for any help you can provide..

Jason Keltz
jas at cs.yorku.ca