Ha! Not really very good at all TT. What with a max of about 3.5A on the bigger ones. But they don't work like a typical mag, as there's a lighting armature with a simple 2 part commutator, as well as a primary and secondary winding relying on the lighting armature as an inductor for the sparks, and a series of tweaks (centrifugal self-locking lighting armature cut-out, a compensating winding comprising half the low tension ignition circuit, etc) designed to get round the inherent weaknesses of the design.
To be avoided unless originality is absolutely essential I'd say.
The original M-L leaflet (600.1 from Sept 1930 is the one I have) is full of laughable statements implying you get the best of both worlds, simplest thing ever made etc. But when economy was the watchword, and when they were new . . . maybe they were OK. The simple bar magnets sure as heck need to be as saturated as they can be got! They claimed to be OK on 500s, outfits etc etc - but quite honestly, a bit of a development blind alley I'd say, although there are a good few still, out there (a lot of them on small Beesas of those days it has to be said).
Re the original question, you won't get lights out of a typical mag, No. I had a chap who swore blind that his grandfather had used a twin cyl mag on a WW1 vintage single, with the second pick-up being used to produce some sort of glimmer of light - but I'm darned if I can see quite how and wouldn't want to try. I just don't see how you'd get enough current out of the thing without putting it in harm's way. But maybe there are clever tricks out there, and with LEDs etc, who knows?
