I think you 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 also found that many antivirus engines certified by OPSWAT at http://www.opswat.com/certified
I hope this helps.
A bit of clarification: Steve was looking for details about McAfee's Security Innovation Alliance SDK for building extensions and products directly on ePO. OPSWAT has great open-market tools, but is unrelated and will not let you build an app in ePO itself (merely gain some external control of endpoint products).