I did indeed! It was Straken, a novel about Colonel Straken of the Imperial Guard. I enjoyed doing it, and it was definitely an interesting experience to write for someone else. Things have changed at Games Workshop over the year, and I've moved onto other projects. Straken is available in hardback and for download, but I'm not sure if any when it's coming out in paperback.
I would say that Dan Abnett is in an interesting and unique situation: he's so firmly established as a writer in the 40k world that his books probably won't change in style unless he wants them to, which I suppose is a way of saying that he can do things that less-established writers wouldn't be allowed to do. But I suppose that doesn't really related to Black Library's way of releasing his books.