3-way SBMV could be made using two incandescent filaments and one arctube - the MV arctube could stand the dimming down to 40% quite well, so filaments could be designed to provide 40+60% of the full arctube power (so e.g. 50+75W for 100-150-250W lamp).
But the problem would be with many 3-way switches: The HID would need guaranteed Break-Before-Make action to avoid extinction when switching between the lowest and medium setting.

The proper warmup (required for proper dimmed HID run up) could be made using bi-metal contact inside the outer shorting both phase inputs together when cold and open only when the arctube warm properly up, so the thing would always start and warmup on full power.

I have seen a fluorescent lamp during the EOL throes sling a red hot glowing filament into the glass and fracturing it and SSSSS! So re enforcing the stem doesn't always prevent vaccum loss....

How about a SOX lamp with mercury instead of sodium.  Maybe at a higher pressure like the old medium pressure mercury lamps.

How about a SOX lamp with mercury instead of sodium.  Maybe at a higher pressure like the old medium pressure mercury lamps.

So you get germicidal (or fluorescent, if you add the phosphor) lamp...