[Aanson]

I'm pretty sure that I've found a good way of getting everything I need now, using Ravagernl's suggestion.

local frame;
for i = 1, NUM_CHAT_FRAMES do
  frame = _G["ChatFrame"..i];
  frame:HookScript("OnEvent", function(self, event, ...)
    self.lastEvent = (self.isDocked and MONITORED_EVENTS[event]) and event;
  end);
  hooksecurefunc(frame, "AddMessage", function(self, message, r, g, b, ...)
    if (not self.lastEvent) then return; end
    -- do my thing...
  end);
end

That way I have a guarantee that the message will actually be displayed. It should be reliable because self.lastEvent will always be obtained immediately before AddMessage gets called.

Cheers all