Hassan, although it did not answer mine or your request, which are very similar or really the same, it was certainly a good reference for me in knowing that someone is indexing their instance using APIs vs the typical web crawler.
While this team really prefers to do a traditional indexing, more so for expected performance purposes, I really like the idea of a federated search instead.
With a federated search, the external solution (search-engine or otherwise) is not trying to concern itself with replicating the security trimmings of the source system.
It leaves that to the source system to perform natively.
I hope we get a response from within this community as I believe these SMEs would certainly know if it has been done before.
Thanks for reaching out Hassan. If you hear/see something elsewhere in the JC, please keep this post in mind.
FYI: The Jive Pointer (formerly known as Jive SharePoint Connector) actually has such federated search built into it's solution. This federated search is actually bi-directional as well. So it's possible, but I am wondering if it has been done outside the context of the Jive Pointer module, which could be leveraged by any 3rd-party solution, not just MS SharePoint.