Indeed, Eric. On isitreal, Markus Brandes in his study of her autograph tells that there is a reference in her biography about having signing groups at her home, often up to six at a time.
I also remember having read this in her daughter's book "Mommie dearest", where Christina reveals that Joan did so because she wasn't good at spelling. But of course, you have to be careful with that.
At least, her spelling on the postcard from the canteen that Eddy posted (thanks so much for having a look again, Eddy! Your revised impression relieves me a lot!) is impeccable.