I think you guys should check out http://www.opswat.com/ there are 2 or 3 products that may be a match. I think that OESIS Framework at http://www.opswat.com/products/oesis-framework provides a single interface to many antivirus packages. Another option is, I think, Metascan at http://www.opswat.com/products/metascan which is more for ISV.

I hope this helps.
Regards,