It's worth remembering that the Start-o-matics were actually called "lighting plant". Back in the 1930s, when they were designed, there were very few electrical devices - desk fans, lightbulbs and radiant heaters being the main ones I can think of, maybe toasters. All of these, when switched on, could consume enough DC current to trigger the generator. Try that with a smartphone charger!
Even as more modern equipment came along - TVs, wireless sets, phonographs etc. - these all had basic transformers & actual on/off switches, which meant they would consume some DC power to trigger the generator, and when switched off would consume nothing.
IMHO the best way to recreate a "SOM" setup now would be with a battery bank and sufficient inverter(s) to power your load requirements. Then sense the voltage on the batteries, and trigger the generator when that drops below a threshold; and switch off when it rises above a different threshold. As Lithium batteries get cheaper and more prevalent, this will IMHO become a fairly normal thing to do. Add in solar panels, micro hydro, a wind turbine, etc., and you could save yourself a few pennies on the old electrickery bill... whether it'd ever pay for itself, is another matter altogether. Although frankly, I'd do it just for the fun of it...