[Exim] ClamAV running not as Exim user

I'm setting up a new server, I'm trying to isolate the various daemons
from each other as much as I can. I'd prefer to have ClamAV running as
user clamav and Exim as exim. Both users are members of the daemon
group (so they can write .pid files and such).

I'm using the exiscan patch for Exim. I set the mode of my exim spool
directory to 750, and changed the group to daemon, and do the same for
the scan directory under that. If I su to clamav I can indeed access
the exim/scan diretory.

But I still get errors about ClamAV not being able to read the file in
the scan directory. I have a feeling the file is being created with a
umask of 0077.

Is there anyway to set the umask for the files to be placed in the
exim/scan directory? That is and not effect the mask for the the other
spool directories.

The only umask option I see for Exim/exiscan is for the pipe transport.
I don't mind patching source if someone can point me in the right
direction.

Thanks.

--
Chris