I spent some time and here it is: https://expirebox.com/download/83bb227f658b3f611e4cf37feb01cd36.html
It's not a backport but it's rather a very simple addon that "merges" guild chats.
There is only one option that you need to do before running the game, and it's to open "GuildXChat.lua" with a text editor (notepad for example) and change the first line to your channel's name (e.g. local guildChat = "myGuildChat";). After that, you can redistribute inside your guilds.
Have a nice day.
Edit (08/01 - 16h10): Some changes were made in the code.
There is a nice WoW API syntax highlighting package for the Atom editor.
This site links some basic guides, most of them should be applicable to TBC as well: http://wowwiki.wikia.com/wiki/WoW_AddOn
www.wowace.com allows you to browse addons by patch.
http://wowwiki.wikia.com/wiki/World_of_Warcraft_API?oldid=1432245 for the old API documentation.
https://www.lua.org/manual/5.1/manual.html for language documentation.