I was trying to do what i think is a simple edit of the single player skirmish game lobby to change what the autofill button does to fill all 'hard' opponents instead of 'normal'.

I extracted lobby.lua and created a mod using Sorian's example mod. Placing the LUA in the hook/lua/ui/lobby folder.

Everything looks right to me, but I can't get any changes to register.  Here is the relevent section of the lobby.lau

        function p_map.btnAutoFillSlots.OnClick(btnself)
            import('/lua/singleplayerlaunch.lua').RepopulateNames()
            for i = 1, LobbyComm.maxPlayerSlots do
                if not self.GUI.slots[i].closed and not self.gameInfo.PlayerOptions[i] then
                    #Play Sound AI Demigod selected PLACEHOLDER
                    PlaySound( 'Forge/UI/snd_ui_generic_menu_click' )

                    local AIName = import('/lua/singleplayerlaunch.lua').GetRandomName()
                    local lobbyName = LOCF('<LOC lobui_0517>AI: %s (Hard Test)', AIName)
                    local randomHero = HeroUtil.GetRandomHero(self.gameInfo)
                    self:HostAddPlayerImpl(i, self.hostID, AIName, false, randomHero, lobbyName)
                    self.gameInfo.PlayerOptions[i].Difficulty = 3
                end
            end
        end

My changes highlighted. 

I looked at the add hard AI from the drop down to change the values above:

elseif key == 'addhardai' then
            diff = 3
            AIName = import('/lua/singleplayerlaunch.lua').GetRandomName()
            lobbyName = LOCF('<LOC lobui_0517>AI: %s (Hard)', AIName)

... END

I am wondering if the game always uses its internal version of this file, so the game lobby cannot be modded?