If they want to do that, why not just get some people hired to work as baggage handlers? On the day in question, just throw the explosives into one of the air cargo containers and be done with it.
I don't buy your idea simply because there are way too many things that could go wrong. First off, how would the explosives and detonators react in a caustic environment? They would also need to know airline procedures for how the 'septic tank' is maintained, etc.. If they have access to that, they have access to better ways to do it and wouldn't need to go through a wondering musician parade on the plane.
Hmm So in your world terrorists only attack the way you might think they attack? No other possibilities? I think I was very clear my thoughts were just a theory. Your thoughts are.... well .... interesting. I am curious in what other contexts in your life do you think this way?