Have a look at this ballast:

http://www.ebay.com/itm/ADVANCE-IMH-50-A-BLS-BALLAST-120-277V-FOR-35W-OR-50W-METAL-HALIDE-LAMP-NEW/151227646990

Does anyone think the built-in ignitor could be removed or at least disabled somehow? If it were possible, I think this would be an awesome ballast to run a 50 watt MV on. Small, electronic (I assume low frequency), and switchable between a 39 watt and 50 watt MH. And there's quite a few on eBay, as well as versions just for 39 or 50 watt lamps.

If you will be able to open the ballast case, the ignitor should be possible to disable.

You may even eliminate it externally using a "250VAC" (so 400..600V clamping voltage) MOV...

But the problem may be the MV characteristics: These ballast tend to check the lamp operation and when detecting an anomaly (the voltage develop differently than usual with the rated MH), shut down.
But you will have to try it out...

I do prefer  Metal Halide bulb better than  High Pressure Sodium for one thing.