Using a pressure cooker would be impractical, as it cannot be opened to put an item in. (Besides, the water would not reach an actual boil – safely. Furthermore, at higher altitudes, even a standard pressure cooker cannot keep water's boiling point above 190°F.) Leaving the item on a platform while the water heats, later to be shaken down, would present a problem of steam in an enclosed space.
Would one have to forgo Hagalah altogether, and default to Libun Kal?