Bob,
Some time ago I also looked into setting up a Postfix server with ClamAV as a filtering system and from what I could tell the general scheme was to create an routing loop (a bit like an AUX send/return on a music mixing desk for the musically inclined) from postfix out to ClamAV, and from ClamAV out to Postfix input again. Postfix supports the passing of data through external services and back.
The way you set this up is completely configurable and should be transparent to any email user be it a sender or a recipient.
I believe that you configure the routing loop seperately for inbound and outbound email.
Now I have not actually done this, so please receive this information with caution, but I don't see why there whould be any problem as long as Aolserver contacts the correct mail agent it shouldn't even know that ClamAV is present.
You may find the following helpful:
General info:
http://www.linuxjournal.com/article/7778
Specific info:
http://www.section6.net/wiki/index.php/Setting_up_Postfix_Spamassassin_Amavisd_Clamav
Regards
Richard
