MOSFETS don't have the same voltage drop characteristics as a P-N junction device such a diode (or BJT) and the rough equivalent is the resistance between source and drain when the device is 'on', given as Rds in the spec sheets. Because the drain-source acts like a resistor the voltage drop is dependent upon the current flowing through it. In the case of guitar FX that's pretty low.
One problem with TO-92 packages is the resistance is usually quite high (several ohms) compared to a medium-power device (much less than 1 ohm). Plus, SMD has taken over, though a medium-power SMD MOSFET is fairly easy to handle.