If you're going to try to implement lean cruise via other means (not HP Tuners or other commercial software), the concept is that you can only go lean when the engine is warmed up and under light load. You don't want to go lean during cold startup (we've seen how this works with E85 conversions) and higher load.
We considered using an EFIE (connects between the O2 sensor and the ECM to go lean), but again you don't want to do this during startup (many EFIE's DO have slowed initial startup) or during any kind of acceleration, AND you have no way to add additional spark and cam advance.
This leaves only a custom tune with hundreds of hours of logging and tweaking. Our Volterado team has 3 active tuners (10 members) and I was driving gas to work every day and driving countless logging runs (unnecessary miles logging/tweaking a tune). Did I mention I charge for free so can drive EV for free so any time spent on gas is out of pocket?