I just did this a few weeks ago. The lounge in JFK was no problem using my phone's "card".
At LHR on the way back, the lounge agent gave me a small amount of grief because there wasn't a date on it (there was, she just missed it). When I pointed this out, she was very apologetic. She was going to let me in anyway.
My status didn't show up on my boarding pass, the check-in agent hand-wrote it.
The Clubhouse at LHR is incredible!