I think the MOSFET thingie is some sort of attempt at adding a level-dependent DC shift to the signal. That brings out even harmonics when the following stages are heavily overdriven, which should make it sound more tube-like, as explained in the famous Russell O. Hamm paper on tube distortion.
That's the only explanation I can think of, since the 22M resistor is too big for it to have any waveshaping or clipping effect directly. Unless that 22M were a misprint (accidental or deliberate)
PS. I used to have the rack-mount Rockman with its MIDI and 100 blinking lights, and I seem to remember the overdrive sounding fairly crappy. I used it with an old solid-state PA amp and a 4x12". I remember there were LEDs inside near the input jack that flickered with the drive. When I think about it, I probably should have kept it, I bet it would rock with a stereo tube power amp.