The assembly would necessarily be in the lavatory so that it couldn't be seen. Unless they passed the object from one person to another out side the lavatory, they would have to leave it inside the lavatory for the next person to bring his part of the device, be it gun or explosive or whatever.
Could it be an EMP (electromagnetic pulse) device designed to disrupt the planes controls and cause a crash?
Much simpler than that: Start a fire, or set off enough of an explosion. That's all it takes. Other posters have already suggested how easy it is. Liquid explosives, by the way, are not detectable by the bomb scanners. Cheddar chesse, on the other hand, sets off the red lights.