There probbaly isn't alot you could do, most of the efficient driving is done in closed loop (at part throttle, the ECU is reading the O2 and trying to keep the AFR at 14.7. The tuning is done changing the fuel tables in open loop (full throttle) and here you would want to be very careful leaning the engine out as you could fry the engine pretty quickly. Though not an expert in modifying Honda ECus, i don't believe you can change the closed loop AFR target.
I've always wondered, with all the work done to the Honda ecu's has anyone disassembled the ecu to the point of releasing a commented disassembly? I mean, I'm sure they can read the chip and look at the raw code, but has anyone gone through it and found what every piece of code does?
All of this makes me wonder whether mileage could be improved on, say, a B-series swap by the use of a P07 ECU. From what I have seen in the P07 that I got withmy D15Z1, there actually IS a space available for a chip. Of course, the ECU would need to be chipped to run a P30, P72, or any custom program AND the EGR would need to be defeated. BUT, I would think that the more precise fuel control offered by the use of the five wire oxygen sensor could result in more efficient operation. Lean burn could even be used for further fuel savings, at least on lower compression DOHC VTEC motors like the B17A and B18C1. But even on higher compression B-series swaps, the early California ECU (which uses a five wire sensor, but no lean burn) could still net savings due to better fuel control, and with no loss in performance.