Honestly with the mere fact that for a flight to count as a flight the pressure differential has to exceed 2 PSI. Any metal exposed to mercury (by a passenger) is likely going to weaken in a radial fashion from point of contact, which means in a pressurized cabin, it shouldn't take long before the air pressure ruptures the leak and the plane would be forced to land due to depressuring.

I doubt this would do serious harm to a plane before landing. I doubt if this would even matter in an unpressurized cargo hold because I was always told it evaporated quite quickly at low pressures.