While the best made cat can create dust as it ages (vibration, heat-cycling, etc) and there can be flow pulses in parts of the exhaust system, even if it was a lot of dust, the flow through the system would push the dust away from the engine. The pulses are much too small (and usually localized near the valve/manifold) to pull anything from the cat to the heads. But I'm no engineer, so prove me wrong.
