Assuming that the internals of an AR10 and AR15 are identically the same, it can be done. I believe that they at dimensionally different, so this thread is a total exercise in mental masturbation. But if you found a registered auto sear for sale, it can legally be installed in a post ban gun. Gun becomes a title three firearm and is exempt from all prohibited dark features. Auto sears are running at about $4000 each. Option number two is spend $6000 on a complete sear gun with a select fire mechanism (if those even existed prior to the 86' ban), cannibalize the gun and install the parts in your post ban gun.
Bear in mind that the 86' ban does not completely prohibit the manufacture of a "new" guns. A sear that was already registered prior to the ban can be installed in any gun that it will fit. Sear is the registered item and not the gun.