Don't know any thing about a NOSE on a BMW but.
You really don't need to remove the altenator.

If you disconnect all the electrical leads on the Alt and don't ground to pully or altenators frame you should be safe.

You can simply disconnect any thing else you are worried over or unground it. (unbolt it and put insulating material between it and the car)

Bear in mind when you are placing your ground that electricty follows the path of least resistance and copper is a better conducter than steel.

i agree w/ RonG...the path of least resistance...we've welded on trucks and boats in my shop w/o disconnecting anything and they've driven out of the shop just fine. we only ground to the frame of the vehicle. we've welded on the aluminum engine block of a boat motor to seal a crack...to my knowledge everything on that boat still works just fine. i think it all depends on what you're welding on and where you ground it.