If you have the flex-fuel feature pack, you should be able to feed an on/off 5v signal into the ECU instead of a flex-fuel sensor and switch directly between the two fuel/timing maps (instead of interpolating between them based on ethanol %). Should be able to tune the base map for pump gas, then tune the flex-fuel map for whatever your alternate fuel is (E85/Race Gas, etc), and flip a switch when you change over. Would involve some dyno sessions and a competent Nistune tuner, but you'd need that with a standalone anyway.