A method that worked for me in terms of updating without having to go thru any twists was simply to update the clamav packages from the volatile database as is indicated to be the best idea on the Clamav website.
Then I went over to the /opt/clamav/bin and /opt/clamav/sbin directories, moved the old executables (renamed), and simply made links to the updated clamav binaries (clamd, clamdscan, freshclam, clamscan).
I then stopped and restarted zimbra, and lo and behold, I believe with no other changes zimbra is now using version .88 of ClamAV.
I'd love to hear if people see any issues with this? I felt this way I can now rely upon the Debian updates and with just links hook in new versions. Freshclam and clamd both seem to be operating fine.
I guess I could have also just copied in the executables under /opt/zimbra, but then I'd lose the nice feature if I update clamav again on my system from debian packages.
Perhaps I am just lucky that doing this doesn't break anything since the configs all stayed the same. But I certainly saw no errors.