Yes, it's difficult to prove because it acts over a period of time along with the difficulty of finding a measure of "creativity", plus you also need a control group which, given the requirements, is difficult.

There's been research on damage or dysfunction of the gland but its effect on visualisation / creativity rests mostly on anecdote and consensus. There are a few people about who've relied on visualisation (for creative stuff) and suffered failure of the pineal (a brain tumour can do it) and reported a loss of these faculties. You know me. When I was warned about fluoridation I accepted it was poison but the claimed effects? I had to delve into it a bit more....