There have been bombs in toilets on aircraft before where, despite a hole in the fuselage, the aircraft structure has been strong enough to enable the aircraft to be landed safely. <Quote> "They build 'em strong at Boeing" <Unquote>
Probably the best known examples of bombs going off in an aircraft that subsequently landed safely are the British built BAC1-11s in the Phillipines.
Another security measure not yet tried is to remove toilet doors!