An importer could bring in guns in bond, destroy the receivers and then reassemble guns using newly made semi-auto receivers that were never on machineguns, plus legal length barrels if necessary. Then they could be removed from bond and sold without registration. (The rules on number of U.S. parts would still apply.) I don't know of anyone who is doing that at present, but it should be legal. (Century Arms did essentially that with M1 rifles which could not be imported, except they left the receivers physically in Canada.)
Jim