I made something similar last year: A p2p chat popup (that sits on bottom right of the page as a bubble) that allows all visitors to chat with each other. It had simple keyword based moderation in-built (can be easily bypassed though).

Was planning to add github oauth to get a known identity and persistent messaging so visitors can chat with each other across sites.

Instead of a webmaster adding script to their site, it was a browser extension.

The intent was two folds:

1. Get to know other people having similar interests,

2. Try something on the lines of a decentralized chat/messaging system.