I wonder on the flip side if any other governments would want to operate pre-clearance here in the states? I doubt any US gateway would have the real estate or the space for say an EU pre-clearance center, but it sure would make life easy. I'd much rather pre-clear EU countries here than arrive at 3-4am Eastern Time and be groggy...or at least deny entry to me here before having to fly 7hrs.
I know the US-Canada preclearance agreement has provisions for Canadian border preclearance facilities in the US - though those have never been implemented. It doesn't really make sense since I can't think of any commercial airports in Canada that don't have full customs facilities available.