There were two Luftwaffe attacks I know of on US Coast Guard cutters proper (not counting attacks on USN vessels manned by the Coast Guard). In operations off eastern Greenland USCGC Northland fired on a 'Junkers' July 10 1943, and USCGC North Star traded fire with another recon a/c north of Jan Mayen Island, claiming to damage and possibly down it, July 23 1943. Obscure incidents to be sure, but by any chance can someone give German accounts, or if not what unit(s) were likely to have been involved?
Background on Greenland Patrol
http://www.uscg.mil/History/h_greenld.html
Joe