First of all, I do not consider this an issue, upon which an official stance (other than: do as you wish) would be taken and has to be taken. This is because in contrast to many other roleplaying games, not all CoC scenarios happen in the very same universe. There are two reasons for this: First, some entities being present in the same universe are certainly contradictory (just think of the several alien and servitor races who would be at open war with each other). Second, there simply is no need for one consistent universe, since certain aspects of the universe are only meant to be known by characters in very small proportions, and thus conflicting information or ideas about the nature of the world can hardly arise (there is a very small risk, if a character »switches« to a keeper who fails to ask about his former Mythos experiences). As a result, if H.P. Lovecraft exists in the universe of a certain scenario, he may explicitly not exist in another one, e.g., a modern-day scenario where certain internet investigations would be bound to find his work.
This being said, I know of some publications, in which H.P. Lovecraft and even CoC are considered to exist:
- In Unseen Masters the characters can visit a Comic Book Shop where they may spot a roleplaying game called »Dark Worlds« by Peter Dannseys and L.S. Isinwyll published by Entropium, which grants 1 % Cthulhu Mythos and takes 1 San.
- In the German pulp CoC setting Der Hexer von Salem, H.P. Lovecraft plays an important role, which exceeds his role in our world.
I would be surprised, if this already was the extensive list.