Hi Jordan,
The Hexanon 57mm f/1.2 is supposed to be an excellent lens and if you look in the archive Dmitri had some pictures with it using his extensively mirror shaved 5D. It looked good as I remember, but was quite similar to the Rokkor 58mm f/1.2, which if I recall you liked fairly well, but ultimately sold. Keep in mind the Hexanon is 460 grams so a pretty big lens.
As I think of getting a GH-2, my first choice for a fast 50ish lens would probably be the nFD 50 f/1.2L. From samples I have seen with that lens, it is quite nice even wide open and it only weighs 380 grams. That lens and the Canon 55m f/1.2 Aspherical and the Zeiss 55mm f/1.2 anniversary lens are I believe the only 50s with a floating element which should improve close up performance, which I think is important for micro 4/3rds. It also has an aspherical element which may help a bit with wide open performance, again on micro 4/3rds wide open performance is likely to be pretty important. I also like the bokeh of from the lens.
But there are others strong contenders too. Including:
Pentax A mount 50mm f/1.2 which only weighs 345g and may be the sharpest of all the fast 50s. (the K mount version might not be a bad choice either but weighs a bit more at 385grams). The bokeh in my view is a little less than optimum on this lens perhaps, however.
Minolta MD 50mm f/1.2 which only weighs 310 grams and is very sharp. I have seen a number of nice shots with this lens from sebboh, but it does tend to have bright rings in the wide open bokeh.
Olympus OM 50mm f/1.2 which is weighs just 285 grams and has fairly nice bokeh, IMO--but with bokeh these things are very subjective.
Nikon 50mm f/1.2 AIS - which weighs 365 grams and is not a bad performer
nFD 50mm f/1.2 - which does not have the aspherical element or the floating element of its L brother, but still seems to be a nice lens and is smaller than the 50 f/1.2L at 315 grams.
Olympus Pen 42mm f/1.2 - which weighs just 255g. This lens looks very nice from the samples I have seen.
There may be others that are worth considering as well, but these are the ones I have thought of. I have left off rangefinder lenses, because at this point it is not clear, to me at least, how well they work at this wide of an aperture on micro 4/3rds. If you had a ton of money the Noct-Nikkor 58mm f/1.2 (which has an aspherical element) or the Zeiss anniversary 55mm f/1.2 (which has a floating element) would likely be excellent, but I doubt you are think of spending that type of money.
Anyway, I hope this helps as you think about fast 50ish lenses. Keep us updated about your pursuits.