Both solutions (custom chat, forking the chat modules) are unnecessary. Just use the Lua API that Roblox provides for its Chat system. This will save you work and keep you automatically up-to-date with any changes made to the Chat scripts.
local ChatServiceRunner = game:GetService("ServerScriptService"):WaitForChild("ChatServiceRunner")
local ChatService = require(ChatServiceRunner.ChatService)
local MainChannel = ChatService:GetChannel("all")
MainChannel.WelcomeMessage = ""