Supposedly "air breaks down at about 30 kV/cm." Automotive ignition systems use higher voltages. If I assume 25.000V per mm:
8mm, 200kV (CC roof to piston face estimate)
40mm, 1MV (cylinder bore radius)
80mm, 2MV
I would guess air density alters the equation somewhat. Humidity does. Water injection not only for charge cooling but for increasing the conductivity of charge air, may be something to consider. If some crazy fuel supplier were to formulate a more conductive form of gasoline, that may also help jump the gap.
Oil formulation is free, maybe they could add gold, silver or copper powders fine enough to aerosol in the sump gases for engine ingestion (that now-banned practice).