The Led doesn't have a big enough electrical resistance. It will work, but it will work differently and it will sound weird, but here it goes:
I have a led light as a charge light on my BMW too. As I said, the resistance isn't big enough to work as the normal light works. Everytime I start the bike I have to rev it once (i usually wait until the engine is warm). The light will go out once you've revved it once (4000/5000 rpm) and it will charge just like the normal system. Believe me, it works every time. Been riding like that for years now.