I do have a suggestion on the thermodynamically disfavored process of changing the water temperature rapidly though. Could you simply hook up one or two other reservoirs? Drain the 'cold' tank to one of those, preheat the other, then pump the other in and suddenly you're cooking.
Or just accept hot/cold faucets with a once-through system for rapid changeovers, followed by precision temp control on recirculation.
I do have a suggestion on the thermodynamically disfavored process of changing the water temperature rapidly though. Could you simply hook up one or two other reservoirs? Drain the 'cold' tank to one of those, preheat the other, then pump the other in and suddenly you're cooking.
Or just accept hot/cold faucets with a once-through system for rapid changeovers, followed by precision temp control on recirculation.