Regardless of what strains Orval uses to do their primary fermentation, there is indeed Brett in the bottle dregs. They don't pasterize or centerfuge the yeast out, so it's chock full of all the organisms that make that beer taste great.
However, Orval dregs stepped up in a starter will not make you an exact Orval clone it seems, or so I have read. I guess their fermentation process is complex. Nonetheless, Orval dregs will make a delicious beer. They are typically added in the secondary, after primary fermentation with a normal Sach strain.