What happens to the water temperature whilst the pump overrun is running? Why do you believe what is happening is insufficient?
My boiler (Vaillant 438) had approximately 3m of pipe between flow/return through an ABV (had, as I now have a LLH, but the pipe run is still relatively short). The idea was to keep the water circulating and allowing it to slowly cool down. Even on such a short run, the temperature drop would be quite dramatic initially.
Stress on the hex is caused by the delta T of flow and return being too high. In the pump overrun scenario, the flow and return temps are roughly the same.
Maybe I've got it all wrong - but sounds like you're trying to solve a problem which doesnt exist.