Think about it, the onboard is much, much slower and uses system RAM for memory. Adding it to a Crossfire equation just seems to me that it would be more of a hinderence to a faster GPU than much help. Although I have no experience with the new Hybrid Crossfire at all, it just seems to me that this is more marketing hype than anything to sell motherboards.
But, it surely doesn't hurt to try. Why don't you let us know what your results are when you do the build? Would be interesting to see how this works first hand.