Does it help when submitting to either an agent or a publisher to have your own marketing plan? Especially for a niche market or subject specific book? And at what point do you mention this plan? In the query. After an agent has already shown interest?
For nonfiction, a marketing plan is necessary to submit to an agent or publisher. But for fiction, it isn’t necessary. Now it’s great to have a marketing plan ready. And it’s pretty ambitious to even have one! I wouldn’t, however, submit it with your query letter.
Only submit the materials that the agent or publisher requests i.e. follow the submission guidelines. Once you have a book deal, hold on to your marketing plan for when you’re in touch with the marketing team (which will probably be a few months before your book is released depending on the publisher). After they present their plan to you and ask if you have any questions/comments, then you can chime in with your marketing ideas.
Let me be real for a sec: Your publisher may not be able to do everything you’d like them to. Their marketing plan will heavily depend on the marketing budget, your book, and a plethora of other factors.
With that being said, there’s nothing wrong with asking or trying. Expect the worst, hope for the best. And don’t get discouraged in case you hear “no.”
Keep planning ahead.