Subjective evaluation of flicker is highly variable, because things like incandescent bulb wattage and other factors play a role. Some people can see flicker much more so than others.
The SOM flywheels improve flicker only as much as my fast correcting AVR does... based on measurements with my low pass filter and an oscilliscope. With my AVR I can now tolerate my 250 watt light/heat lamps for my shop which previously gave me partial seizures. I can switch back and forth on my AVR to stock harmonic or AVR via 1 switch, and I made an RC low pass filter so I could measure the 5.5 Hz variation in AC peak voltage via oscilliscope. It is dramatically improved.
A responsive AVR could also now be applied to the SOM, something not very practical at the time of their design. This would add that much more to voltage regulation, and I suspect even I, with epilepsy, could tolerate 60 watt bulbs with both SOM flywheels and my AVR with mains excitation.
For most people, just changing to LED lighting which regulates LED current via a small switching supply will totally eliminate flicker. This is the simplest solution for people who can tolerate LED lighting. I can't.
As for B&D's suggestion of big electrolytic capacitors on the excitation side- sounds good but absolutely not helpful. If "smoothing" the excitation current was going to work, there would already be no problem as the rotor has huge inductance (a few henries) and is a highly effective filter in itself. The underlying problem is that the engine speeds up during the power stroke, and slows down during compression. This causes a big change in voltage out with a constant excitation current. Because the frequency is varying, this requires large and rapid changes in the excitation current to try and maintain voltage output. This is complicated by the lag in response by the rotor's inductance. The correction is always too late, so only so much can be done without a much more complex electronic solution. (A microcontroller would be required to anticipate the correction- a fairly complex bit of real time control.)
The simple solution is to switch your lights to something self regulating. If you must use incandescent bulbs, then a fast AVR and/or heavier flywheels/generator pulley ala the SOM is what you need. The smaller the bulb wattage, the harder it will be to get the flicker down to tolerable. The two tools available are adding rotation mass and electronic voltage regulation.