Promotion from your clients to rank higher

Hi!
I just need your opinnion. So I make a website development/resell and just wondering what you think about that:

I have some business clients who don't use blog on their site.
I wonder, when I set up a site for them, is it good to post one article about my site with my main keywords in it. Just like web 2.0 but I use my clients pages. I'm not going to list it on the website - just let it be in post category. No menu button so his/her clients can't find it but search engine can.
Or is it too unethical.
When we sign a contract I can put this into contract, that we can use one page for promoting and client get 5-10% off orsmthing..

Thanks

it is quite unethical, but I think clients will be happier with a link on their footer which will do wonders if multiplied

Yeah we just made our website and I put link into footer to two sites. Ancor texts were different. We dropped page 4 to 10 with our main kw.. :S
I removed the links and used disallow tool yesterday so soon I know was that the problem. I think we dropped because suddenly there where arount 60 links to our page just from 2 sites.