You are reading waaaaayyyyy too much into this. She is the most well known author around these days. She has written a series of books many in his audience will have read. She backs up the message of the speech, work hard and good things will eventually happen.
He may or may not know of her politics, it really has nothing to do with him mentioning her in the speech. Her politics really have little to do with why he is holding her up as an example to follow.
Oh, I'm certain he knows of her politics!
http://vodpod.com/watch/1491426-barack-obama-reportedly-gets-j-k-rowlings-autograph
http://blog.taragana.com/n/obama-tells-j-k-rowling-he-is-harry-potter-fan-24964/
"LONDON - US President Barack Obama has revealed that he is a huge fan of the Harry Potter series, and even told its author J K Rowling about it.
Obama, 47, who was genuinely pleased to meet 43-year-old Rowling, told her at the end of the Downing Street G20 dinner that he had read all her books, and that he also read them to his daughters Sasha and Malia.
[...]
Rowling had been among a number of notable women from the arts, business, and sport world who were being entertained at Downing Street by Gordon Brown’s wife Sarah. (ANI)"
And, Mrs. Obama sat next to J.K. Rowling at dinner:
http://www.thefirstpost.co.uk/people,2155,michelle-obama-wins-over-londoners-and-gets-a-royal-hug-at-the-palace,80099
"The big question was whether the First Lady would bare those famous arms. For the visit to Buckingham Palace, she chose to cover up in a black jacket and white top. But by the time she attended last night's Jamie Oliver-cooked dinner in Downing Street, where she sat between J K Rowling and Dame Kelly Holmes, she had indeed changed to a sleeveless ivory top."
Gee, I wonder who made that seating arrangement? And, I'm pretty darn sure Michelle had plenty of time to talk with Rowling while waiting for her full course meal.
Perhaps I could be reading waaaaayyyy too much into it; but, having lived amongst communists and socialist in academic and private settings, believe you me, they all know each other and keep in constant touch. Not only that, but they are continually endorsing and piling accolades upon each other to ingratiate themselves amongst an unsuspecting public.
Cheers