I love the idea behind your plug-in. I was able to get it to work, well, kind of. The issue is that no other widget can be placed below your Secondary HTML Content widget(s). Doing this screws up the entire pages alignment.
Also, the Secondary HTML Content boxes (where you actually put the content) must be treated as a page in WordPress. I am using the AddThis Social Media Plug-in, and if I put one or five Secondary HTML Content widgets on a page, the AddThis plug-in inserts to those areas.