We looked at this, and have chosen to partner with Groundspring.org, an organization with common values and methods to Benetech, to implement this functionality on Bookshare.org. They do a good job of delivering both the security we require and the Bookshare.org look we desire. If it works out well, we'll expand it to our other projects and Benetech itself. If you make a donation, let me know how it went. Customer feedback is critical!
Bookshare.org - Donate Now page.