Well it all goes back to the 12025(f) thing. That code specifically says that belt holsters are exempt. But it doesn't say only belt holsters are exempt. So there isn't any law that specifically says wearing a gun and mags around your neck is illegal. Wouldn't be overly surprised if you'd get arrested for it from some cop who reads 12025(f) thing as being an "only" rule though. I'm guessing a DA wouldn't bother with it, but then what do I know?
I can't imagine it being very comfortable or very practical. Especially with that bayonet of yours!