LH is strictly enforcing "no card, no fun". I learned this the hard way by CO's failure of sending the cards abroad in time. They don't care about the *G printed on your boarding pass at all, so this is not bringing you in. There are exceptions, however, quite rarely.
However what worked for me was the replacement membership card print out available at the CO page. Just bring this print-out with a date printed on, discuss for a minute, and enjoy the lounge. Also works at the SEN lounges.
I called up UA before a trip and they emailed me a letter addressed to LH attesting to my Status with UA and that the card was put into the mail but wasnt gonna reach me in time, both @ IAD & FRA I was let into the LH Lounges
didnt try to get a guest in with me so I dont know what they would have done or said if I had tried