Just wanted to post a heads-up to interested parties, the stub page on MSDN for IOpenSearchSource and its one method have been replaced by the draft documentation.
The new page roughly sums up its use and purpose:
A client-side OpenSearch data source that sits in between the Microsoft Windows OpenSearch provider and the external data source. With a search connector (a .searchconnector-ms file), Windows Explorer calls your implementation with the query parameters. Your implementation returns results formatted in RSS or Atom format. That allows your implementation to provide custom authentication UI and connect to the data source using its proprietary API.
More details at: http://msdn.microsoft.com/en-us/library/dd378289(VS.85).aspx
Unfortunately we still don’t have any details on how you actually hook up an implementation of this. The doc / SDK folks are working on this but it’s part of a larger effort that’s taking a bit of time. In the meantime I’m going to see if I can post an example here or on CodePlex. Most likely I’ll be posting a detailed .NET example followed by a simpler C++ version.
If you have questions about whether this API may end up being useful to you, let me know, and hopefully I can help clarify what is or isn’t possible.