Page 180 in Fate is the Hunter makes reference to boffins getting the crews to operate at reduced RPM and MP so I imagine that operating LOP was part of it. EK Gann found that it was bollocks in practise, but then again what would he know.
I haven't read Fate is the Hunter yet. (It's on my "to do" list) however I have read a number of articles about how Charles Lindbergh demonstrated lean of peak operation to greatly extend the range of navy aircraft operating in the Pacific.
I wondered why quite a number of pilots rejected the method until I read their reason for refusing to use it. It turns out it was because they were operating in an active war zone and were on high alert as soon as they were airborne even when the target of the day was a considerable distance away.
They weren't interested in stuffing around with mixtures when they needed instant power to escape a nasty situation or press on with an attack when they spotted a stray enemy aircraft.
The average GA aircraft doesn't have the Navy's wartime problems so it makes sense to run your engine where it's happy, cool and giving you the best bang for your buck in the fuel and maintenance department.