Clearly you're lying, or delusional. Whoever can write a bbs can write a decentralized publically editable bbs. It may take slightly longer, so it does depend on your deadline. Of course, if your deadline is so soon as to not allow you enough time to design it, that's a bad course. There are two software development styles: one where there's a about a week for just thinking about what you would want it to be, a few weeks designing it, and a few (slow, boring, yet necessary) days, perhaps a week or two, typing out the code and proofs, and doing all the other tests, too; and one where there's about a week of furiously typing away, possibly testing some parts of the system, followed by at least a few months, probably a few years, perhaps decades, of frustrating error search and correction. It's up to you which way you'll develop software. I recommend the former. Many companies stupidly insist on the latter.
One idea for a prototype is to use a publically readible, publically commitable, version-control repository, where each regular file is a post or the index (which specifies which versions of which posts, and how they're organized under boards, threads,,), and a script which takes the choice of which version of the index, and which interface form (e.g. HTML, PHP, or whatever other language), and generates the interface using the index, and whatever requisite data, as the bbs. The main page of the web-site can be an interface/index selector/editor, with sensible defaults.
The main work would be in writing the script which formats the data to the specified interface.
To satisfy that requirement, allowing a PHP interface would probably be sufficient.