I am loooking for a solution:

There are some websites using different CMS or only static HTML pages.
I would like to open a forum (or a guestbook that is simplier but forum have more topics) that content is shared among the different websites. Every user of that website can read or post the forum on his local website. The design of the forum must be locally stored so the forum design is like the website design but the content is similar.
I hope you understand what I mean.
I don't need complex forum but easy usage. No attachment or similars. One thing is important, every website's owner should moderate only his user's post.

Do you have any idea how to solve it?