You might be in trouble. Even with conformal coating on the PCB, the IPC spec is 14.53mm at 5000 volts. (You require even more spacing at higher altitudes). You may not get arcing, but you might get leakage currents that can cause problems.
I must confess i am not interested in any safety standards etc.......i am just interested in whether i will see faulty circuit operation......i am wondering if i can just smother the pad in silicone or RTV?