You could try the 'tricks' like putting a resistor (eg 100 Ohm) in the screen connection.
That will increase the voltage drop of any screen current flowing, causing a larger interference signal at the receiver (i.e. more hum at the pedal). You would probably have better results decreasing the impedance between pedals to keep the voltage induced by noise currents as low as possible, e.g. use cable with heavy braid shield, and/or add additional copper braid straps between pedals.