Here is what he said to me about his campaign at the Nightwatch of Lankhmar group on Facebook:
Jay Hafner at the Nightwatch of Lankhmar group wrote:Im currently running dnd 5e Princes of the Apocalypes south of Karteshla. Ive run so nuch stuff in the cities of Warhammer, that it is kinda second nature to use Nehwon. I just reskinned all of the dnd race stats to Nehwon races. Ive got a house rulebook if youre interested.
Obviously, I said I was interested, as I wouldn't be starting a topic about it here, if I wasn't.
He was kind enough to put his: Lankhmar: Hafner Campaign House Rulebook online.
It's seven pages long and has a list of useful WotC documents at the end of the booklet. If you are thinking of running a Lankhmar campaign using 5th Edition Dungeons & Dragons rules, you should definitely look at this. In fact, it might be useful even if you are not using 5e.