This sounds like a really neat approach! I certainly use writing as therapy, because I think that vulnerability is the key to unlocking self-acceptance, and there is nothing like getting feedback that helps you look at things from different angles. If my life was not the hectic mess it is at the moment (I have just moved country with a family of 4) I would definitely sign up.

Also I have used a degree of writing with my patients but it has always been focused on goal setting and completion of tasks, things like SMART goals and the CBT box with Thoughts, Emotions, Physical symptoms and behaviour. We keep it concise and short because many of my patients have either verbal or written (motor & cognitive) communication problems. I like it because having to be succinct leads to conversations looking for the right words that go in quite deep and help me understand the perception they have of their situation. But even for the more able ones I have never thought of writing prose. I am now getting into learning more about Acceptance and Commitment therapy and I can see how a written approach would aid that sort of process. When you get that workbook done I would love to get a copy. Good luck with it.