If you are bussed, you are left into the B area, so no problem there. If you are using a jetway that is further away from the B area (e.g. B3) you'll have to show your BP to enter airside: yes, a BP for a non-Schengen area will let you through, and if you're asked why you go there, just tell the security agent that you want to visit the LH lounge.