[dcb]

Sure it is doable, a basic arduino or clone can handle it if you want to make it easier to get started. You will get the most bang for your buck by focusing on the O2 sensor IMHO, see also EFIE. Some folks have just added (subtracted?) an offset voltage to lean out the mixture, even on the narrow band sensors, but you could also hold the high signal longer or the low signal longer with the microprocessor if you wanted more precise control.

your o2 sensor cannot handle much current (an analog voltmeter used to measure its output can fry it) and is somewhat expensive though, so proceed with caution. And you already know (or will research) all the caveats of running your engine too lean.