I remember this being done years ago.... but , here's the catch, it only fired a FEW select bulbs ... by few i mean 1 bulb that i can remember, it was a 6500k bulb as well. It would not fire all MH bulbs, and sorry to say , i do not believe MH bulbs come with caps built in, I have yet to see one that does.
MH ballasts usually come with the core, igniter and cap... M57's usually core and cap
A m57 will only run a handful of bulbs, and that is with a cap. Not sure what the non cap version can run. Either way it's a good find