I had a cross-server chat on my old community’s servers. One of the other admins made a LUA addon that would send chat messages starting with specific characters to a PHP page I wrote (having GET parameters with the server it came from, the player who sent it and the message itself). This page would also display messages and messages ID from all the servers, so the addon could get the page content and show the messages in the chat, keeping the last ID in memory to avoid duplicates.
There was some delay before every server received the messages as the addon was set to read the page’s content each 5 or 10 seconds to avoid overloading the web server. Also we only had 3 servers, I don’t think it would have worked that smoothly if we had a lot more.
I don’t think the addon was using any kind of modules however, so it could be one way of doing what you want if you haven’t been able to get one of the above ways to work.