One thought is that the TDT collective should vary the box probabilities very very slightly, so that Omega can tell which has the lowest probability, but regular CDT agents can't

Works easily if the TDT agent has a specific CDT agent in mind. It simply determines what to do by running a simulation of the CDT agent and letting that influence it's decision. Then the CDT agent is causally trapped, and can't compute what the TDT agent will decide because it can't identify "its own decision" within the TDT agent.