Being a libertarian, I am completely honest with all my business-transactions, business-offerings and partnerships.
From a marketing perspective, would it be beneficial to post my business-plan on the website where it's been implemented?
The short answer: no. Not unless your potential customers are all venture capitalists and MBA types. For most products, the typical customer won't care or even if they do, they won't understand. They just want to know, as quickly as possible, how you're going to help them or entertain them.
Business plans tend to have a lot of business jargon and legalese, and it will just get in the way for the most part.
Instead, spend the bulk of your time optimizing your website to provide an awesome user experience. That's what will help you, from a marketing standpoint.
I can honestly appreciate your desire to be open and upfront with your customers and potential customers, though. I think it is a good idea. And I don't think that desire should be totally discarded.
There will be a few customers who will want to know more about who you are and what you do. As your site develops, rather than your full business plan, it may be a good idea to post something more readable, for those customers who are interested in knowing how you tick. For example, look at 37signals. In the middle of their page, under the title "We built the company that we’d want to do business with. We hope you do too," you'll see a list of eight basic principles that guide their company. Perhaps something like that might be useful to your customers.
And then, if you really want to give customers your business plan, have a link to it in a place like that.
Only as a learning tool for others, and viral activity to your brand. Then yes!