Good discussion around an interesting topic. I'll drop in my 2 cents/pence worth.
I'm not convinced creativity is centrally either left or right brained. To be honest, I see people arguing like this as influenced by a culture which is responding to the idea that emotions are weak by attributing creativity and the arts to the more emotional aspect of ourselves in an attempt to strengthen them. In my mind, this is not helpful, because in seeking to strengthen the perception of emotions and emotionalism, we actually wind up limiting the arts and reinforcing the Platonic distinction between the philosopher and the poet. Yet, I see no good reason why the philosopher (or, an intellectual who relies on logic and reason) cannot be one and the same as the poet (or an artistic person who connects with and stirs the emotions).
Rather, I see creativity as connections made between formerly disparate items, and this could be either left or right brained. Where our brains tend to have these connections is on the paths frequently traveled. For me, I see bursts of creativity in my main job (fundraising, marketing, etc.) and in my hobbies (philosophy, theology, writing), and especially where these two intersect (public speaking). So, it this theory is correct, the best way to cultivate creativity in writing is to write while adding in new information and experiences which can be connected through writing.
And yet, I am reminded of the advice of one of my university professors, who said, "Those who try to be creative will inevitably fail at it." This has borne out in my life; when I was trying to create something new and unique, I found that someone had already come up with everything I invented. But, when I stopped worrying about being original and just focused on telling the story, I started improving as a writer and storyteller.
So, my thought on the matter is twofold. We ought to recognize that creativity can happen with all types of people, and not just the conventionally "artistic" sort, and some people may actually be more creative when there are rules in place, rather than restricted. Second, we ought to stop trying so hard to be creative, and just work on telling stories, so that creativity can naturally develop.