Having seen the artwork for the various grail diaries, the Nine Gates books, countless maps, etc. I don't think creating accurate artwork is the issue. There's too much talent here to make this all that difficult to pull off.
From my understanding the real problem is the Necronomicon's creator - I've heard he'll go after anyone that tries to replicate his books - cover or artwork. I do believe some members on this forum have experience dealing with Mr. Sullivan in that very arena.
Not saying it's good or bad on Sullivan's part. As the artist, he has every right to protect his creation. I'm just saying that from my understanding, if you try to replicate his work - for whatever reason or purpose - he will come after you.