[p3lim]

You mean ctrl-clicking the mob in the world?
Anyways, just use the normal loot event LOOT_OPENED and check for IsControlKeyDown(), which when pressed/depressed triggers MODIFIER_STATE_CHANGED.
To prevent default auto-looting, use the ctrl key as the modifier to avoid autolooting.

Something like this perhaps:

local Frame = CreateFrame('Frame')
Frame:RegisterEvent('LOOT_OPENED')
Frame:SetScript('OnEvent', function(self, event, arg1, arg2)
    if(event == 'MODIFIER_STATE_CHANGED') then
        if((arg1 == 'LCTRL' or arg1 == 'RCTRL') and not arg2) then
            -- loot everything
            arg1 = true
            self:UnregisterEvent(event)
        else
            return
        end
    end
 
    if(arg1 and not IsControlKeyDown()) then
        LootFrame_InitAutoLootTable(LootFrame)
        LootFrame:SetScript('OnUpdate', LootFrame_OnUpdate)
        LootFrame.AutoLootDelay = LOOTFRAME_AUTOLOOT_DELAY
    else
        LootFrame.AutoLootDelay = 0
        LootFrame.AutoLootTable = nil
        self:RegisterEvent('MODIFIER_STATE_CHANGED')
    end
 
    LootFrame.page = 1
    LootFrame_Show(LootFrame)
 
    if(not LootFrame:IsShown()) then
        CloseLoot(not arg1 or IsControlKeyDown())
    end
end)